GitHub Actions require re-running in order to be successful

[lukemartinlogan]

For some reason, github actions will fail within 5 minutes due to not being able to find pkg-config. Something is buggy with the github action cache. It's strange, but it makes actions take much longer to run as it has to completely re-install all dependencies for each push and commit.

[hyoklee]

@lukemartinlogan, I updated the Developer's Guide.
See also PR #504.

[lukemartinlogan]

The CI now installs spack packages separate from building Hermes. This results in more consistent and correct caching of packages.