Automated snapshot builds for ramips (and others) are stuck in Buildbot since Nov 25

Seems like a trivial error, but it has failed in a few targets with similar file lock errors

fatal: Unable to create '/builder/ramips_mt7621/build/.git/index.lock': File exists.

Another git process seems to be running in this repository, e.g.
an editor opened by 'git commit'. Please make sure all processes
are terminated then try again. If it still fails, a git process
may have crashed in this repository earlier:
remove the file manually to continue.