git: force-fully ignore the environment variable GIT_VERSION

While it used to be possible to override the version shown in `git
version` via `make GIT_VERSION=...`, the same was not true of the
environment variable `GIT_VERSION`.

However, that changed in Git's 992bc5618f2 (GIT-VERSION-GEN: fix
overriding GIT_VERSION, 2024-12-20).

To force the build process to derive the version shown in the output of
`git version`, let's forcefully un-set the environment variable during
the build.

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

Author: dscho

mingw-w64-git/PKGBUILD

@@ -164,6 +164,9 @@ build() {
   CPPFLAGS="$(echo "$CPPFLAGS" |
 	sed 's/-D__USE_MINGW_ANSI_STDIO\(=[0-9]*\)\?//')"
 
+  # Guarantee that `GIT-VERSION-GEN` determines the version
+  unset GIT_VERSION
+
   # Make git-bash.exe first, to avoid triggering `* new link flags`
   rm -f *.res &&
   make -f ../mingw-w64-git.mak git-wrapper.exe \