Add a pidlock for build_JuliaInterface()

[lgoettgens]

Resolves #1249.

This is more meant as a bandaid. Once this is merged, I'll open a new issue on the point of the redundant rebuilds as mentioned in #1249:

(However that would still end up re-compiling JuliaInterface multiple times. To protect against that, more work is needed -- but perhaps it is not necessary given that this should not affect release versions of GAP.jl, so it's just for our internal use?

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 76.89%. Comparing base (97ce797) to head (0c545cb).
⚠️ Report is 3 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1250      +/-   ##
==========================================
- Coverage   76.91%   76.89%   -0.02%     
==========================================
  Files          60       60              
  Lines        4778     4783       +5     
==========================================
+ Hits         3675     3678       +3     
- Misses       1103     1105       +2     

Files with missing lines	Coverage Δ
pkg/JuliaInterface/src/calls.c	41.40% <ø> (ø)
src/setup.jl	97.43% <100.00%> (-1.70%)	⬇️

... and 1 file with indirect coverage changes

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[lgoettgens]

I know that this now unnecessarily touches the juliainterface c code (thus failing the treehash job), but since I want to merge #1248 immediately afterwards anyway, I would just leave it in (instead of restarting CI once more).