Propagate conda override variables to docs build in CI
This MR fixes an oversight introduced by Set CONDA_OVERRIDE_ARCHSPEC for Linux x86_64 (!1455 - merged) where the docs
CI build didn't receive the same CONDA_OVERRIDE_ARCHSPEC
variable as the other jobs, so then couldn't render the environments for the documentation.
The fix is to have the docs
CI job inherit from the .linux-64
template that is used to provide the overrides in the first place.
Edited by Duncan Macleod