Adds a .gitignore to keep the lib directory (Fixes: #40004)

1 job for master in 1 minute and 50 seconds (queued for 1 minute and 14 seconds)
Status Name Job ID Coverage
  Test
failed test #17740

00:01:50

 
Name Stage Failure
failed
test Test
Processing triggers for libgdk-pixbuf2.0-0:amd64 (2.38.1+dfsg-1) ...
$ git submodule init
Submodule 'src/build' (https://gitlab.torproject.org/tpo/metrics/base.git) registered for path 'src/build'
$ git submodule update
Cloning into '/builds/tpo/metrics/collector/src/build'...
Submodule path 'src/build': checked out '3fb743daabb97439027396915f815aeb3ec720a0'
$ mkdir lib
mkdir: cannot create directory 'lib': File exists
ERROR: Job failed: exit code 1