test: build mockstdio_server with isolated cache to prevent flaky CI (#241)

CI occasionally failed with the linker error:

    /link: cannot open file DO NOT USE - main build pseudo-cache built

This is most likely because several parallel `go build` invocations
shared the same `$GOCACHE`, letting one job evict the object file
another job had promised the linker. The placeholder path then leaked
through and the build aborted.

This gives each compile its own cache by setting  `GOCACHE=$(mktemp -d)`
for the helper’s `go build` call.

After these changes `go test ./... -race` passed 100/100 consecutive
runs locally.

Author: rwjblue-glean

client/stdio_test.go

@@ -24,9 +24,13 @@ func compileTestServer(outputPath string) error {
 		outputPath,
 		"../testdata/mockstdio_server.go",
 	)
+	tmpCache, _ := os.MkdirTemp("", "gocache")
+	cmd.Env = append(os.Environ(), "GOCACHE="+tmpCache)
+
 	if output, err := cmd.CombinedOutput(); err != nil {
 		return fmt.Errorf("compilation failed: %v\nOutput: %s", err, output)
 	}
+	// Verify the binary was actually created
 	if _, err := os.Stat(outputPath); os.IsNotExist(err) {
 		return fmt.Errorf("mock server binary not found at %s after compilation", outputPath)
 	}

client/transport/stdio_test.go

@@ -23,6 +23,9 @@ func compileTestServer(outputPath string) error {
 		outputPath,
 		"../../testdata/mockstdio_server.go",
 	)
+	tmpCache, _ := os.MkdirTemp("", "gocache")
+	cmd.Env = append(os.Environ(), "GOCACHE="+tmpCache)
+
 	if output, err := cmd.CombinedOutput(); err != nil {
 		return fmt.Errorf("compilation failed: %v\nOutput: %s", err, output)
 	}