[CI] fix /nix dir permissions

Apparently, this is needed to run 'nix' inside tests (e.g. start corenlp)
4 jobs for 483-dev-corenlp-in-nix-flake in 44 minutes and 1 second (queued for 36 minutes and 49 seconds)
Status Job ID Name Coverage
  Cabal
passed #18723
cabal

00:21:01

 
  Stack
passed #18724
stack

00:03:44

 
  Bench
manual #18725
allowed to fail manual
bench
 
  Test
failed #18726
test

00:19:16

 
Name Stage Failure
failed
test Test
Test suite garg-test: FAIL
Test suite logged to:
/builds/gargantext/haskell-gargantext/dist-newstyle/build/x86_64-linux/ghc-9.6.6/gargantext-0.0.7.4.7/t/garg-test/opt/test/gargantext-0.0.7.4.7-garg-test.log
0 of 1 test suites (0 of 1 test cases) passed.
Error: [Cabal-7125]
Tests failed for test:garg-test from gargantext-0.0.7.4.7.

Cleaning up project directory and file based variables
ERROR: Job failed: exit code 1