test: fix e2e commands on Windows

Instead of trying to make this work nicely cross-platform,
just push the Coverage logic into the GitHub Actions job,
as that's really where we care about it.

(It's surprisingly difficult to make this nicely portable;
to make PowerShell not error out if the path does not exist
you have to ignore ALL errors and the way that env vars are
passed to processes is not the same.)

Signed-off-by: Milas Bowman <milas.bowman@docker.com>

.github/workflows/ci.yml

@@ -179,7 +179,11 @@ jobs:
         name: Test plugin mode
         if: ${{ matrix.mode == 'plugin' }}
         run: |
-          make e2e-compose
+          rm -rf ./covdatafiles
+          mkdir ./covdatafiles
+          make e2e-compose GOCOVERDIR=covdatafiles
+          go tool covdata textfmt -i=covdatafiles -o=coverage.out
+
       -
         name: Test standalone mode
         if: ${{ matrix.mode == 'standalone' }}