ci: clear and mark MAKEFLAGS exported just once

Clearing it once upfront, and turning all the assignment into
appending, would future-proof the code even more, to prevent
mistakes the previous one fixed from happening again.

Also, mark the variable exported just once at the beginning.  There
is no point in marking it exported repeatedly.
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

ci/lib.sh

@@ -74,6 +74,9 @@ check_unignored_build_artifacts ()
 	}
 }
 
+# Clear MAKEFLAGS that may come from the outside world.
+export MAKEFLAGS=
+
 # Set 'exit on error' for all CI scripts to let the caller know that
 # something went wrong.
 # Set tracing executed commands, primarily setting environment variables
@@ -101,7 +104,7 @@ then
 	BREW_INSTALL_PACKAGES="git-lfs gettext"
 	export GIT_PROVE_OPTS="--timer --jobs 3 --state=failed,slow,save"
 	export GIT_TEST_OPTS="--verbose-log -x --immediate"
-	export MAKEFLAGS="--jobs=2"
+	MAKEFLAGS="$MAKEFLAGS --jobs=2"
 elif test -n "$SYSTEM_COLLECTIONURI" || test -n "$SYSTEM_TASKDEFINITIONSURI"
 then
 	CI_TYPE=azure-pipelines
@@ -126,7 +129,7 @@ then
 	BREW_INSTALL_PACKAGES=gcc@8
 	export GIT_PROVE_OPTS="--timer --jobs 10 --state=failed,slow,save"
 	export GIT_TEST_OPTS="--verbose-log -x --write-junit-xml"
-	export MAKEFLAGS="--jobs=10"
+	MAKEFLAGS="$MAKEFLAGS --jobs=10"
 	test windows_nt != "$CI_OS_NAME" ||
 	GIT_TEST_OPTS="--no-chain-lint --no-bin-wrappers $GIT_TEST_OPTS"
 else
@@ -185,4 +188,4 @@ GIT_TEST_GETTEXT_POISON)
 	;;
 esac
 
-export MAKEFLAGS="$MAKEFLAGS CC=${CC:-cc}"
+MAKEFLAGS="$MAKEFLAGS CC=${CC:-cc}"