x/build/cmd/coordinator: store CL number in the query parameters

[kevinburke]

If the build completes, you get this message:

TryBot result not found (already done, invalid, or not yet discovered from Gerrit). Check Gerrit for results.

If we stored the CL number in the URL (for example commit=deadbeef&cl=123456), and we couldn't find the commit number, we could assume the build is complete and either redirect you back to it, or provide a link to the right CL from the complete farmer build.

[toothrot]

That page could definitely be more informative. This sounds related to #10313 (comment).