build: reenable building arm64 concurrently (#30626)

build/ci.go

@@ -205,12 +205,6 @@ func doInstall(cmdline []string) {
 	// Configure the build.
 	gobuild := tc.Go("build", buildFlags(env, *staticlink, buildTags)...)
 
-	// arm64 CI builders are memory-constrained and can't handle concurrent builds,
-	// better disable it. This check isn't the best, it should probably
-	// check for something in env instead.
-	if env.CI && runtime.GOARCH == "arm64" {
-		gobuild.Args = append(gobuild.Args, "-p", "1")
-	}
 	// We use -trimpath to avoid leaking local paths into the built executables.
 	gobuild.Args = append(gobuild.Args, "-trimpath")