Fix oom when running unit tests in CI (#5284)

* fix oom when testing

Signed-off-by: Alex Goodman <wagoodman@users.noreply.github.com>

* right-size CI test runners

the `r` family and `ram=32+128` were sized for the tmpfs footprint rather
than the workload. with the caches back on disk the build+test peak fits in
16GB, so move to `m`/`c` at `ram=16+32`.

- ~27% cheaper per instance-hour on spot, and a much deeper pool to schedule
  against (~20 x86 types vs 7)
- `cpu=4+8` with `ram=16+32` lets price-capacity-optimized opportunistically
  land an 8 vCPU box when it's cheap
- plain `volume=100gb` replaces the provisioned throughput. a 4 vCPU instance
  caps near 150MB/s of EBS and gp3's free baseline is already 125MB/s, so the
  override bought nothing

Signed-off-by: Alex Goodman <wagoodman@users.noreply.github.com>

---------

Signed-off-by: Alex Goodman <wagoodman@users.noreply.github.com>

.github/actions/bootstrap/action.yaml

@@ -47,7 +47,17 @@ runs:
         APT_PACKAGES: ${{ inputs.bootstrap-apt-packages }}
       run: |
         IFS=' ' read -ra packages <<< "$APT_PACKAGES"
-        DEBIAN_FRONTEND=noninteractive sudo apt update && sudo -E apt install -y "${packages[@]}"
+        # the AMI already ships most of these, and `apt update` costs ~40MB of index (minutes,
+        # when archive.ubuntu.com throttles us) to find that out. only touch apt if we must.
+        missing=()
+        for pkg in "${packages[@]}"; do
+          dpkg -s "$pkg" >/dev/null 2>&1 || missing+=("$pkg")
+        done
+        if [ ${#missing[@]} -eq 0 ]; then
+          echo "all apt packages already present: ${packages[*]}"
+          exit 0
+        fi
+        DEBIAN_FRONTEND=noninteractive sudo apt update && sudo -E apt install -y "${missing[@]}"
 
     # ORAS cache: restore-only on non-default branches / forks
     - name: Restore ORAS cache from github actions