[gimp] build: fix a weird (yet unexplained) build bug.

[Jehan <jehanp src gnome org>]

commit 7cca69cd49d4b685ae41fba84822e6fedd8c109c
Author: Jehan <jehan girinstud io>
Date:   Wed May 5 02:12:52 2021 +0200

    build: fix a weird (yet unexplained) build bug.
    
    Without this, the native Windows GIMP build fails in CI with:
    
    > make[2]: *** No rule to make target '_install\include\gegl-0.4\gegl.h', needed by 
'../libgimpcolor/gimpadaptivesupersample.lo'.  Stop.
    
    Even though `gegl.h` is present (as checked through artifacts). It's
    just so weird.

 build/windows/gitlab-ci/build-gimp-msys2.sh | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)
---
diff --git a/build/windows/gitlab-ci/build-gimp-msys2.sh b/build/windows/gitlab-ci/build-gimp-msys2.sh
index b261a41b6e..0ffb8a837c 100644
--- a/build/windows/gitlab-ci/build-gimp-msys2.sh
+++ b/build/windows/gitlab-ci/build-gimp-msys2.sh
@@ -69,7 +69,12 @@ pacman --noconfirm -S --needed \
     mingw-w64-$MSYS2_ARCH-vala \
     mingw-w64-$MSYS2_ARCH-xpm-nox
 
-export GIMP_PREFIX=`realpath ./_install`
+# XXX We've got a weird error when the prefix is in the current dir.
+# Until we figure it out, this trick seems to work, even though it's
+# completely ridiculous.
+mv _install ~
+
+export GIMP_PREFIX=`realpath ~/_install`
 export PATH="$GIMP_PREFIX/bin:$PATH"
 export PKG_CONFIG_PATH="${GIMP_PREFIX}/lib/pkgconfig:$PKG_CONFIG_PATH"
 export PKG_CONFIG_PATH="${GIMP_PREFIX}/share/pkgconfig:$PKG_CONFIG_PATH"
@@ -90,3 +95,6 @@ make -j4
 make install
 
 ccache --show-stats
+
+# XXX Moving back the prefix to be used as artifacts.
+mv "${GIMP_PREFIX}" .