GHA test coverage: increase webfakes startup timeout

Because covr makes the app very slow. This won't be
needed with the next version of webfakes, which will
use a longer timeout when running in covr, by default.

.github/workflows/test-coverage.yaml

@@ -34,6 +34,8 @@ jobs:
             install_path = file.path(normalizePath(Sys.getenv("RUNNER_TEMP"), winslash = "/"), "package")
           )
         shell: Rscript {0}
+        env:
+          R_WEBFAKES_PROCESS_TIMEOUT: 30000
 
       - name: Show testthat output
         if: always()