Remove sudo from GH workflow

Turns out the docker command in CI is set up such that it doesn't need
an explicit sudo, so remove it.  This will also allow us to easily pass
environment variables to mktree in the next commit (as otherwise we'd
have to use the ugly -E, --preserve-env option).

No functional change.

.github/workflows/linux.yml

@@ -9,8 +9,8 @@ jobs:
     steps:
       - uses: actions/checkout@v3
       - name: Build the Fedora testing environment
-        run: sudo ./mktree.docker build
+        run: ./mktree.docker build
         working-directory: ./tests
       - name: Run the test suite
-        run: sudo ./mktree.docker check --interactive=false -j$(nproc)
+        run: ./mktree.docker check --interactive=false -j$(nproc)
         working-directory: ./tests