goma_link: Use the python executable to invoke goma_ld.py

Otherwise the script doesn't run on Windows; relying on the shebang
doesn't work there.

Bug: 877722
Change-Id: Id7665fb1d03bece0d015eb6a9f2f7fbf96325d64
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2582078Reviewed-by: Nico Weber <thakis@chromium.org>
Reviewed-by: Bob Haarman <inglorion@chromium.org>
Commit-Queue: Hans Wennborg <hans@chromium.org>
Cr-Commit-Position: refs/heads/master@{#835839}

build/toolchain/gcc_toolchain.gni

@@ -196,6 +196,7 @@ template("gcc_toolchain") {
       # number of jobs, but this results in too low a value on
       # Chrome OS builders. So we pass in an explicit value.
       goma_ld =
+          "$python_path " +
           rebase_path("//tools/clang/scripts/goma_ld.py", root_build_dir) +
           " --gomacc ${goma_path} --jobs 200 -- "
     } else {