ci: use twice the CPU count on both providers

GitHub Actions sets JOBS to ten regardless of runner size, while
GitLab CI uses the detected CPU count. Use twice the CPU count for
Make and prove on both providers, doubling GitLab's job count.

Five GitHub Actions attempts per policy, with the long tests enabled,
gave these sums of per-job median successful build/test-step times
(minutes; four or five samples per job) [1-3]:

                      Fixed 10   CPU count   2x CPU count
  Linux Make             278.9       273.9          259.9
  macOS Make              94.5       119.1           99.8
  Windows Make           102.2       103.8          100.8

Workflow overhead is excluded; Windows runner images varied.

Use twice the CPU count to scale concurrency with runner size while
avoiding the larger macOS slowdown observed with one job per CPU.
Compared with ten jobs, this trades a lower Linux total for a higher
macOS total.

Keep GitLab's Linux and Windows CPU queries. On macOS, use the native
sysctl command on both providers so CPU detection does not depend on
GNU coreutils.

Link: https://github.com/tamird/git/actions/runs/36070869894/attempts/1 [1]
Link: https://github.com/tamird/git/actions/runs/36070867524/attempts/1 [2]
Link: https://github.com/tamird/git/actions/runs/36070867647/attempts/1 [3]

Assisted-by: LLM
Signed-off-by: Tamir Duberstein <tamird@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

ci/lib.sh

@@ -226,7 +226,6 @@ then
 	cache_dir="$HOME/none"
 
 	GIT_TEST_OPTS="--github-workflow-markup"
-	JOBS=10
 
 	distro=$(echo "$CI_JOB_IMAGE" | tr : -)
 elif test true = "$GITLAB_CI"
@@ -242,7 +241,6 @@ then
 	case "$OS,$CI_JOB_IMAGE" in
 	Windows_NT,*)
 		CI_OS_NAME=windows
-		JOBS=$NUMBER_OF_PROCESSORS
 		;;
 	*,macos-*)
 		# GitLab CI has Python installed via multiple package managers,
@@ -252,11 +250,9 @@ then
 		export PATH="$(brew --prefix)/bin:$PATH"
 
 		CI_OS_NAME=osx
-		JOBS=$(nproc)
 		;;
 	*,almalinux:*|*,alpine:*|*,debian:*|*,fedora:*|*,ubuntu:*|*,i386/ubuntu:*)
 		CI_OS_NAME=linux
-		JOBS=$(nproc)
 		;;
 	*)
 		echo "Could not identify OS image" >&2
@@ -283,6 +279,19 @@ else
 	exit 1
 fi
 
+case "$CI_OS_NAME" in
+windows|windows_nt)
+	JOBS=$NUMBER_OF_PROCESSORS
+	;;
+osx)
+	JOBS=$(sysctl -n hw.logicalcpu)
+	;;
+*)
+	JOBS=$(nproc)
+	;;
+esac
+JOBS=$((2 * JOBS))
+
 MAKEFLAGS="$MAKEFLAGS --jobs=$JOBS"
 GIT_PROVE_OPTS="--timer --jobs $JOBS"