Accommodate recent spack versions in bash scripts and build instructions

[oparry-ukaea]

Recent spack versions (e.g. 0.22) generate environment builds in $tmp and create links to them at <env root>/build-{architecture}-{hash}/spack-build-{hash}
This breaks some functionality in activate and scripts/run_eg.sh which assume the builds are at <env root>/spack-build-{hash}
The build instructions in README.md probably also need updating.