Retry DownloadLibDatadog (#8014)

## Summary of changes

Add retry logic with exponential backoff to the `DownloadLibDatadog`
build step for Windows to handle transient network failures when vcpkg
downloads dependencies from GitHub.

  ## Reason for change

CI builds occasionally fail with 504 Gateway Timeout errors when vcpkg
attempts to download tools like `7zr.exe` from GitHub during the
`DownloadLibDatadog` step. These transient failures cause unnecessary
build failures.

  Example error from CI:
` error: https://github.com/ip7z/7zip/releases/download/25.01/7zr.exe:
failed: status code 504`

  ## Implementation details

  - Wrap the vcpkg install command in a retry loop (max 3 attempts)
  - Apply exponential backoff between retries: 5s, 10s, 15s
  - Add logging to track attempt numbers and failures
  - Re-throw exception if all retries are exhausted
  - Pattern matches existing `DownloadWafVersion` retry implementation

  ## Test coverage

Existing CI tests will validate the change. The retry mechanism only
activates on failures, so successful builds are unaffected.

## Other details
<!-- Fixes #{issue} -->


<!--  ⚠️ Note:

Where possible, please obtain 2 approvals prior to merging. Unless
CODEOWNERS specifies otherwise, for external teams it is typically best
to have one review from a team member, and one review from apm-dotnet.
Trivial changes do not require 2 reviews.

MergeQueue is NOT enabled in this repository. If you have write access
to the repo, the PR has 1-2 approvals (see above), and all of the
required checks have passed, you can use the Squash and Merge button to
merge the PR. If you don't have write access, or you need help, reach
out in the #apm-dotnet channel in Slack.
-->

Author: NachoEchevarria

tracer/build/_build/Build.Steps.cs

@@ -621,8 +621,33 @@ async Task DownloadWafVersion(string libddwafVersion = null, string uncompressFo
                             var packages = $@"{BuildArtifactsDirectory}\obj\vcpkg\packages";
                             var buildTrees = $@"{BuildArtifactsDirectory}\obj\vcpkg\buildtrees";
 
-                            // This big line is the same generated by VS when installing libdatadog while building the profiler
-                            vcpkg($@"install --x-wait-for-lock --triplet ""{triplet}"" --vcpkg-root ""{vcpkgRoot}"" ""--x-manifest-root={RootDirectory}"" ""--x-install-root={installRoot}"" --downloads-root ""{downloads}"" --x-packages-root ""{packages}"" --x-buildtrees-root ""{buildTrees}"" --clean-after-build");
+                            const int maxRetries = 3;
+
+                            for (int attempt = 1; attempt <= maxRetries; attempt++)
+                            {
+                                try
+                                {
+                                    Logger.Information($"Attempt {attempt}: Running vcpkg install for {triplet}...");
+                                    // This big line is the same generated by VS when installing libdatadog while building the profiler
+                                    vcpkg($@"install --x-wait-for-lock --triplet ""{triplet}"" --vcpkg-root ""{vcpkgRoot}"" ""--x-manifest-root={RootDirectory}"" ""--x-install-root={installRoot}"" --downloads-root ""{downloads}"" --x-packages-root ""{packages}"" --x-buildtrees-root ""{buildTrees}"" --clean-after-build");
+                                    Logger.Information($"vcpkg install succeeded on attempt {attempt}.");
+                                    break; // Exit loop on success
+                                }
+                                catch (Exception ex)
+                                {
+                                    Logger.Warning($"Attempt {attempt} failed: {ex.Message}");
+
+                                    if (attempt == maxRetries)
+                                    {
+                                        throw;
+                                    }
+
+                                    // Exponential backoff before retrying
+                                    var delaySeconds = 5 * attempt;
+                                    Logger.Information($"Waiting {delaySeconds} seconds before retry...");
+                                    await Task.Delay(TimeSpan.FromSeconds(delaySeconds));
+                                }
+                            }
                         }
                     }
                 });