gokrazy: use monorepo for gokrazy appliance builds (monogok)

This switches our gokrazy builds to use a new variant of cmd/gok called
opinionated about using monorepos: https://github.com/bradfitz/monogok

And with that, we can get rid of all the go.mod files and builddir forests
under gokrazy/**.

Updates #13038
Updates gokrazy/gokrazy#361

Change-Id: I9f18fbe59b8792286abc1e563d686ea9472c622d
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

.github/workflows/natlab-integrationtest.yml

@@ -18,7 +18,7 @@ jobs:
         uses: actions/checkout@8e8c483db84b4bee98b60c0593521ed34d9990e8 # v6.0.1
       - name: Install qemu
         run: |
-          sudo rm /var/lib/man-db/auto-update
+          sudo rm -f /var/lib/man-db/auto-update
           sudo apt-get -y update
           sudo apt-get -y remove man-db
           sudo apt-get install -y qemu-system-x86 qemu-utils