If you have an environment like
```
$ cat spack.yaml
spack:
specs: [openmpi@4.1.0+cuda]
```
this PR provides a new command `spack change` that you can use to adjust environment specs from the command line:
```
$ spack change openmpi~cuda
$ cat spack.yaml
spack:
specs: [openmpi@4.1.0~cuda]
```
in other words, this allows you to tweak the details of environment specs from the command line.
Notes:
* This is only allowed for environments that do not define matrices
* This is possible but not anticipated to be needed immediately
* If this were done, it should probably only be done for "named"/not-anonymous specs (i.e. we can change `openmpi+cuda` but not spec like `+cuda` or `@4.0.1~cuda`)
2968ae667f