Cut the dependency on ijar when not stripping the bootclasspath

Rule deps are fetched eagerly, even if unused, so we need to actually cut the edge to ijar when not stripping for bazel bootstrapping to succeed.

Unfortunately, with the way Bazel integration tests are set up, there's no way to test this with Bazel without making a release. I manually verified that `bazel query deps()` reports the right results and the targets build correctly.

```
$ bazel query 'deps(//toolchains:platformclasspath, 1)'
//:license
//toolchains:DumpPlatformClassPath.java
//toolchains:current_java_runtime
//toolchains:ijar
//toolchains:incompatible_language_version_bootclasspath_enabled
//toolchains:language_version_bootstrap_runtime
//toolchains:platformclasspath
//toolchains:utf8_environment
@bazel_tools//tools/allowlists/function_transition_allowlist:function_transition_allowlist
@bazel_tools//tools/jdk:bootstrap_runtime_toolchain_type

$ bazel query 'deps(//toolchains:platformclasspath_unstripped, 1)'
//:license
//toolchains:DumpPlatformClassPath.java
//toolchains:current_java_runtime
//toolchains:incompatible_language_version_bootclasspath_enabled
//toolchains:language_version_bootstrap_runtime
//toolchains:platformclasspath_unstripped
//toolchains:utf8_environment
@bazel_tools//tools/allowlists/function_transition_allowlist:function_transition_allowlist
@bazel_tools//tools/jdk:bootstrap_runtime_toolchain_type
```

(ignore-relnotes)

PiperOrigin-RevId: 828368783
Change-Id: Ie29eea882ad41e4efa8f1dd752679a221a239ae1

Author: hvadehra

toolchains/bootclasspath.bzl

@@ -224,14 +224,16 @@ Rerun with --toolchain_resolution_debug='@bazel_tools//tools/jdk:bootstrap_runti
     )
 
     bootclasspath = ctx.outputs.output_jar
-    _run_ijar(
-        actions = ctx.actions,
-        label = ctx.label,
-        ijar = ctx.executable._ijar,
-        input = unstripped_bootclasspath,
-        output = bootclasspath,
-    )
-    bootclasspath = bootclasspath if ctx.attr.strip else unstripped_bootclasspath
+    if ctx.attr.strip:
+        _run_ijar(
+            actions = ctx.actions,
+            label = ctx.label,
+            ijar = ctx.executable._ijar,
+            input = unstripped_bootclasspath,
+            output = bootclasspath,
+        )
+    else:
+        ctx.actions.symlink(output = bootclasspath, target_file = unstripped_bootclasspath)
     return [
         DefaultInfo(files = depset([bootclasspath])),
         java_common.BootClassPathInfo(
@@ -241,6 +243,9 @@ Rerun with --toolchain_resolution_debug='@bazel_tools//tools/jdk:bootstrap_runti
         OutputGroupInfo(jar = [bootclasspath]),
     ]
 
+def _compute_ijar(strip):
+    return Label("//toolchains:ijar") if strip else None
+
 _bootclasspath = rule(
     implementation = _bootclasspath_impl,
     attrs = {
@@ -261,7 +266,7 @@ _bootclasspath = rule(
             default = "@bazel_tools//tools/allowlists/function_transition_allowlist",
         ),
         "_ijar": attr.label(
-            default = "//toolchains:ijar",
+            default = _compute_ijar,
             cfg = "exec",
             executable = True,
         ),