fixup! release: add Mac OSX installer build

Starting with upstream commit 4580bcd ("osxkeychain: avoid
incorrectly skipping store operation", 2025-11-14), the osxkeychain
credential helper now includes git-compat-util.h and links against
libgit.a.

The osxkeychain Makefile has:

    CFLAGS ?= -g -O2 -Wall -I../../.. $(BASIC_CFLAGS)

The -I../../.. is needed to find git-compat-util.h when building from
contrib/credential/osxkeychain/. However, our config.mak sets CFLAGS
explicitly, so the ?= conditional assignment is ignored and the include
path is never added, causing:

    git-credential-osxkeychain.c:5:10: fatal error:
        'git-compat-util.h' file not found

See https://github.com/microsoft/git/actions/runs/21495314261 for the
failing run.

Fix this by adding a target-specific variable in config.mak that appends
the required include path specifically for git-credential-osxkeychain.o.

Signed-off-by: Johannes Schindelin <Johannes.Schindelin@gmx.de>

Author: dscho

.github/workflows/build-git-installers.yml

@@ -509,6 +509,11 @@ jobs:
           cat >>git/config.mak <<EOF
           CFLAGS = -I$homebrew_prefix/include -I/usr/local/opt/gettext/include
           LDFLAGS = -L"$(pwd)"
+
+          # The osxkeychain Makefile uses CFLAGS ?= with -I../../.. to find
+          # git-compat-util.h, but since we set CFLAGS above, that default is
+          # ignored. Use a target-specific variable to add the include path.
+          git-credential-osxkeychain.o: CFLAGS += -I../../.. \$(BASIC_CFLAGS)
           EOF
 
           # Configure the Git to use the OS supplied libcurl.