CI: install git-filter-repo dependency

Signed-off-by: Stephen Gutekanst <stephen@hexops.com>

.github/workflows/push_subrepos.yml

@@ -13,6 +13,8 @@ jobs:
           # fetch full Git history, otherwise push-subrepos.sh would fail due to not being able to
           # split the git subtree.
           fetch-depth: 0
+      - name: Install git-filter-repo
+        run: sudo apt-get install -y git-filter-repo
       - name: "Push recent commits to subrepositories"
         env:
           ACCESS_TOKEN: ${{ secrets.HEXOPS_MACH_PUSH_SUBREPOS }}