Experiment with go.work in workflows/syncsubdeps.

Trying to avoid go get having to download dependencies. We just want
it up update go.mod and go.sum.

.github/workflows/syncsubdeps.yml

@@ -13,14 +13,24 @@ jobs:
       with:
         fetch-depth: 1
 
+    - name: Set Up Workspace
+      # Submodules shouldn't cause a download.
+      run: |
+        set -o pipefail
+        go work init . $(ls ./deps/*_*/cgo.go | xargs -r dirname)
+
     - name: Update go.mod
       run: |
-        find deps -maxdepth 2 -type f -name libmanifest | \
-          xargs -r dirname | \
-          xargs -rn1 basename | \
-          while read os_arch; do
-            go get "github.com/${{ github.repository }}/deps/$os_arch@${{ github.sha }}"
-          done
+        set -o pipefail
+        submods=(
+          $(find deps -maxdepth 2 -type f -name libmanifest | \
+              xargs -r dirname | \
+              xargs -rn1 basename | \
+              while read os_arch; do
+                echo "github.com/${{ github.repository }}/deps/$os_arch@${{ github.sha }}"
+              done)
+        )
+        go get "${submods[@]}"
 
     - id: info
       name: Extract Information