Introduce cargo-gamma

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Copilot-Session: cd88f2d9-7bc6-448e-bcb7-469f4aaa0600

Author: Martin Taillefer

.cargo/mutants.toml

@@ -4,6 +4,8 @@
 examine_globs = ["crates/**"]
 exclude_globs = [
     "crates/cargo-aprz-lib/**",
+    # Remove when cargo-gamma runs on itself; until then neither mutation engine covers these crates.
+    "crates/cargo-gamma*/**",
     "crates/testing_aids/**",
 ]
 all_features = true

.github/workflows/main.yml

@@ -253,7 +253,8 @@ jobs:
           enable-sccache: true
           rust-toolchain: RUST_NIGHTLY
           rust-components: llvm-tools-preview
-          cargo-tools: CARGO_LLVM_COV_VERSION
+          # Some coverage tests shell out to nextest and require it to be installed.
+          cargo-tools: CARGO_LLVM_COV_VERSION, CARGO_NEXTEST_VERSION
 
       # execute
       - name: Generate Coverage (all-features)