azure-pipelines: parallelise the Windows mingw-w64-git build

The 'Build mingw-w64-git package' step calls `please.sh
build-mingw-w64-git ... HEAD`, which delegates to `makepkg-mingw`
and ultimately to Git's top-level Makefile. With no `MAKEFLAGS` in
the task's environment, makepkg-mingw runs `make` without an
explicit `-j`, which leaves the bulk of the per-file work
(generating the perl scripts, building the doc tree, running the
contrib targets, compiling each .c separately) running serially on
a single core.

Like the SDK sparse checkout (parallelised via `checkout.workers=56`
in the previous commit), most of the wall-clock time here is spent
shuffling small files in and out of the SDK's pacman cache and the
generated build tree, not in the compiler itself; the work is
predominantly I/O-bound, so the right factor is well above the agent
core count rather than `$(nproc)`.

Hoist the previously ESRP-conditional `env:` block on the Windows
'Build mingw-w64-git package' step out of the `${{ if }}`
expression so MAKEFLAGS can sit unconditionally next to the
ESRP-only entries (which now nest inside the same `env:` via a
compile-time `${{ if }}: ESRP_TOOL: ... ESRP_AUTH: ...
SYSTEM_ACCESSTOKEN: ...` insert), and add `MAKEFLAGS: -j15`. A
short comment captures the I/O-bound rationale so a future
maintainer doesn't try to "fix" it down to `$(nproc)`.

Assisted-by: Claude Opus 4.7
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Author: dscho

.azure-pipelines/release.yml

@@ -254,8 +254,16 @@ extends:
                       bash "$apply" "$patches/git-sdk" "/$(mingwprefix)"
                 - task: Bash@3
                   displayName: 'Build mingw-w64-git package'
-                  ${{ if eq(parameters.esrp, true) }}:
-                    env:
+                  env:
+                    # The mingw-w64-git build is heavy on parallel work
+                    # that the underlying compile (and especially the
+                    # contrib + doc + i18n + perl-script generation
+                    # stages) can soak up far more aggressively than the
+                    # core count would suggest, since most of it is I/O
+                    # against the SDK's pacman cache and the make rules
+                    # have very few real serialisation points.
+                    MAKEFLAGS: -j15
+                    ${{ if eq(parameters.esrp, true) }}:
                       ESRP_TOOL: $(ESRP_TOOL)
                       ESRP_AUTH: $(ESRP_AUTH)
                       SYSTEM_ACCESSTOKEN: $(System.AccessToken)