[build] Remove the last absolute paths from RBE builds.

Extend --check-clean (enabled on bots) to verify build commands don't include absolute paths.

The non-RBE GCC and MSVC builds still have absolute paths. GCC doesn't have -ffile-compilation-dir so it uses -fdebug-prefix-map, which is not fixable. The MSVC build has absolute path in the toolchain wrappers, which might be fixable.

TEST=ci
Change-Id: I3b984aaab7aefa7ff527f0a039ca42281224a09d
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/506505
Reviewed-by: Alexander Aprelev <aam@google.com>
Commit-Queue: Ryan Macnak <rmacnak@google.com>

runtime/vm/BUILD.gn

@@ -168,7 +168,7 @@ template("gen_vm_platform") {
           invoker.add_implicit_vm_platform_dependency
     }
     single_root_scheme = "org-dartlang-sdk"
-    single_root_base = rebase_path("../../")
+    single_root_base = rebase_path("../../", root_build_dir)
     libraries_specification_uri = "org-dartlang-sdk:///sdk/lib/libraries.json"
     outputs = [
       "$root_out_dir/vm_platform" + output_postfix + ".dill",