CI(run-python-test-set): don't collect code coverage for real (#12611)

## Problem

neondatabase/neon#12601 did't compleatly disable writing `*.profraw`
files, but instead of `/tmp/coverage` it started to write into the
current directory

## Summary of changes
- Set `LLVM_PROFILE_FILE=/dev/null` to avoing writing `*.profraw` at all

.github/actions/run-python-test-set/action.yml

@@ -181,6 +181,8 @@ runs:
           # Ref https://github.com/neondatabase/neon/issues/4540
           # cov_prefix=(scripts/coverage "--profraw-prefix=$GITHUB_JOB" --dir=/tmp/coverage run)
           cov_prefix=()
+          # Explicitly set LLVM_PROFILE_FILE to /dev/null to avoid writing *.profraw files
+          export LLVM_PROFILE_FILE=/dev/null
         else
           cov_prefix=()
         fi