Add Pre/Release Github Environments

[CodeGat]

Add secrets/variables for Gadi and Gadi Prerelease Github Environments for ACCESS-ESM1.5 that respect the need for restriction of parts of the UM. Note that it will be a different path from the usual models.

Requires resolution of ACCESS-NRI/build-cd#45

[CodeGat]

Secrets and variables for Gadi and Gadi Prerelease have been added.
Note: Spack Release location is at /g/data/vk83/apps/spack/0.21/, which doesn't yet exist. I assume we want to deploy to the most recent location. Furthermore, we have stipulated that the RELEASE_LOCATION is /g/data/vk83/apps/spack/release-restricted-ukmo, which also doesn't exist just yet. Also can change the name if needed.

[CodeGat]

Changed the path to release to $spack/../restricted/ukmo/release based on ACCESS-NRI/build-cd#45 (comment). The appropriate secrets and variables for Prerelease and Release have been set. See https://github.com/ACCESS-NRI/ACCESS-ESM1.5/settings/environments if you have access. Closing.