guix: Expose GIT_COMMON_DIR in container as readonly

When using worktrees or submodules, you'll see a `.git' plain text file
at the root of your working tree instead of the usual `.git' directory.

This plain text file will point to the real GIT_DIR, under the
GIT_COMMON_DIR. From experimentation, the full GIT_COMMON_DIR is
required to exist for operations such as git-archive(1), so we expose it
as readonly inside the container.

contrib/guix/guix-build.sh

@@ -105,6 +105,7 @@ for host in ${HOSTS=x86_64-linux-gnu arm-linux-gnueabihf aarch64-linux-gnu riscv
                                  --pure \
                                  --no-cwd \
                                  --share="$PWD"=/bitcoin \
+                                 --expose="$(git rev-parse --git-common-dir)" \
                                  ${SOURCES_PATH:+--share="$SOURCES_PATH"} \
                                  ${ADDITIONAL_GUIX_ENVIRONMENT_FLAGS} \
                                  -- env HOST="$host" \