Turn gain maps on in ci-unix-shared-installed.yml (#2511)

The default values for the install fix are taken from:
https://cmake.org/cmake/help/v3.31/command/install.html#signatures

.github/workflows/ci-unix-shared-installed.yml

diff --git a/.github/workflows/ci-unix-shared-installed.yml b/.github/workflows/ci-unix-shared-installed.yml
index 2deb81c..27f0487 100644
--- a/.github/workflows/ci-unix-shared-installed.yml
+++ b/.github/workflows/ci-unix-shared-installed.yml
@@ -104,6 +104,7 @@
           -DAVIF_BUILD_EXAMPLES=ON -DAVIF_BUILD_APPS=ON
           -DAVIF_BUILD_TESTS=ON -DAVIF_ENABLE_GTEST=ON -DAVIF_GTEST=LOCAL
           -DAVIF_BUILD_GDK_PIXBUF=ON  ${{ env.CMAKE_AVIF_FLAGS }}
+          -DAVIF_ENABLE_EXPERIMENTAL_GAIN_MAP=ON
       - name: Build libavif with [[nodiscard]] (ninja)
         if: runner.oldest-cmake == 'false'
         working-directory: ./build_nodiscard

cmake/Modules/LocalLibargparse.cmake

diff --git a/cmake/Modules/LocalLibargparse.cmake b/cmake/Modules/LocalLibargparse.cmake
index 870b272..faad62d 100644
--- a/cmake/Modules/LocalLibargparse.cmake
+++ b/cmake/Modules/LocalLibargparse.cmake
@@ -23,6 +23,8 @@
         libargparse
         GIT_REPOSITORY "https://github.com/kmurray/libargparse.git"
         GIT_TAG ${AVIF_LOCAL_LIBARGPARSE_GIT_TAG}
+        # TODO(vrabaud) remove once CMake 3.13 is not supported anymore.
+        PATCH_COMMAND sed -i.bak -e "s:install.*:include(GNUInstallDirs)\\\\ninstall(TARGETS libargparse RUNTIME DESTINATION \\\\$\\\\{CMAKE_INSTALL_BINDIR\\\\} LIBRARY DESTINATION \\\\$\\\\{CMAKE_INSTALL_LIBDIR\\\\} ARCHIVE DESTINATION \\\\$\\\\{CMAKE_INSTALL_LIBDIR\\\\}):" CMakeLists.txt
         UPDATE_COMMAND ""
     )
     avif_fetchcontent_populate_cmake(libargparse)