ci: Unset CI_* variables before run_hadrian and test_make

The goal here is to somewhat sanitize the environment so that
performance tests don't fluctuate as much as they have been doing. In
particular the length of the commit message was causing benchmarks to
increase because gitlab stored the whole commit message twice in
environment variables. Therefore when we used `getEnvironment` it would
cause more allocation because more string would be created.

See #20431

-------------------------
Metric Decrease:
    T10421
    T13035
    T18140
    T18923
    T9198
    T12234
    T12425
-------------------------

1 files changed, 16 insertions, 12 deletions

diff --git a/.gitlab/ci.sh b/.gitlab/ci.sh
index 9a0e940ee7..fd39622726 100755
--- a/.gitlab/ci.sh
+++ b/.gitlab/ci.sh
@@ -447,10 +447,11 @@ function test_make() {
   fi
 
   run "$MAKE" test_bindist TEST_PREP=YES
-  run "$MAKE" V=0 VERBOSE=1 test \
-    THREADS="$cores" \
-    JUNIT_FILE=../../junit.xml \
-    EXTRA_RUNTEST_OPTS="${RUNTEST_ARGS:-}"
+  (unset $(compgen -v | grep CI_*);
+    run "$MAKE" V=0 VERBOSE=1 test \
+      THREADS="$cores" \
+      JUNIT_FILE=../../junit.xml \
+      EXTRA_RUNTEST_OPTS="${RUNTEST_ARGS:-}")
 }
 
 function build_hadrian() {
@@ -541,14 +542,17 @@ function run_hadrian() {
   if [ -z "${BIGNUM_BACKEND:-}" ]; then BIGNUM_BACKEND="gmp"; fi
   read -r -a args <<< "${HADRIAN_ARGS:-}"
   if [ -n "${VERBOSE:-}" ]; then args+=("-V"); fi
-  run hadrian/build-cabal \
-    --flavour="$BUILD_FLAVOUR" \
-    -j"$cores" \
-    --broken-test="${BROKEN_TESTS:-}" \
-    --bignum=$BIGNUM_BACKEND \
-    -V \
-    "${args[@]+"${args[@]}"}" \
-    "$@"
+  # Before running the compiler, unset variables gitlab env vars as these
+  # can destabilise the performance test (see #20341)
+  (unset $(compgen -v | grep CI_*);
+    run hadrian/build-cabal \
+      --flavour="$BUILD_FLAVOUR" \
+      -j"$cores" \
+      --broken-test="${BROKEN_TESTS:-}" \
+      --bignum=$BIGNUM_BACKEND \
+      -V \
+      "${args[@]+"${args[@]}"}" \
+      "$@")
 }
 
 # A convenience function to allow debugging in the CI environment.