Use appropriate indicator for Windows signing determination in build workflow

The "build" workflow signs the Windows builds of the application. The signing process relies on access to GitHub Actions
secrets. For this reason, the workflow is configured to only sign the builds when it has access to GitHub Actions
secrets to avoid spurious failures of the workflow that would otherwise be caused by signing failure.

Previously the signing was determined based on the value of the `github.event.pull_request.head.repo.fork` context item.
That was effective for the use case of the workflow being triggered by a pull request from a fork (for security reasons,
GitHub Actions does not give access to secrets under these conditions).

However, there is another context under which the workflow might run without access to the signing secrets, for which
the use of context item is not appropriate. It is important to support the use of the workflow in forks of the
repository. In addition to the possible value to hard forked projects, this is essential to allow conscientious
contributors to test contributions to the build and release system in their own fork prior to submitting a pull request.
The previous configuration would cause a workflow run performed by a contributor in a fork to attempt to sign the
Windows build. Unless the contributor had set up the ridiculously complex infrastructure required to perform the signing
for the Windows build, which is utterly infeasible, this would cause the workflow to fail spuriously.

The appropriate approach, which has been the established convention in the rest of the workflow code, is to use the
secret itself when determining whether to attempt the signing process. If the secret is not defined (resulting in it
having an empty string value), then the signing should be skipped. If it is defined, then the signing should be
performed.

Author: per1234

.github/workflows/build.yml

@@ -290,7 +290,7 @@ jobs:
       SIGNTOOL_PATH: "C:/Program Files (x86)/Windows Kits/10/bin/10.0.19041.0/x86/signtool.exe"
       WIN_CERT_PASSWORD: ${{ secrets[matrix.config.certificate-password-secret] }}
       WIN_CERT_CONTAINER_NAME: ${{ secrets[matrix.config.certificate-container] }}
-      WIN_SIGNING_ENABLED: ${{ !github.event.pull_request.head.repo.fork }}
+      WIN_SIGNING_ENABLED: ${{ secrets[matrix.config.certificate-password-secret] != '' }}
 
     strategy:
       matrix: