ci: rename "runs_on_pool" to "distro"

The "runs_on_pool" environment variable is used by our CI scripts to
distinguish the different kinds of operating systems. It is quite
specific to GitHub Actions though and not really a descriptive name.

Rename the variable to "distro" to clarify its intent.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

.github/workflows/main.yml

@@ -303,7 +303,7 @@ jobs:
       CC: ${{matrix.vector.cc}}
       CC_PACKAGE: ${{matrix.vector.cc_package}}
       jobname: ${{matrix.vector.jobname}}
-      runs_on_pool: ${{matrix.vector.pool}}
+      distro: ${{matrix.vector.pool}}
     runs-on: ${{matrix.vector.pool}}
     steps:
     - uses: actions/checkout@v4

ci/install-dependencies.sh

@@ -11,7 +11,7 @@ UBUNTU_COMMON_PKGS="make libssl-dev libcurl4-openssl-dev libexpat-dev
  tcl tk gettext zlib1g-dev perl-modules liberror-perl libauthen-sasl-perl
  libemail-valid-perl libio-socket-ssl-perl libnet-smtp-ssl-perl"
 
-case "$runs_on_pool" in
+case "$distro" in
 ubuntu-*)
 	sudo apt-get -q update
 	sudo apt-get -q -y install language-pack-is libsvn-perl apache2 \

ci/lib.sh

@@ -279,7 +279,7 @@ then
 
 	cache_dir="$HOME/none"
 
-	runs_on_pool=$(echo "$CI_JOB_IMAGE" | tr : -)
+	distro=$(echo "$CI_JOB_IMAGE" | tr : -)
 	JOBS=$(nproc)
 else
 	echo "Could not identify CI type" >&2
@@ -318,7 +318,7 @@ export DEFAULT_TEST_TARGET=prove
 export GIT_TEST_CLONE_2GB=true
 export SKIP_DASHED_BUILT_INS=YesPlease
 
-case "$runs_on_pool" in
+case "$distro" in
 ubuntu-*)
 	if test "$jobname" = "linux-gcc-default"
 	then