update spack.yaml for spack v0.19.1

[weixiao-zhan]

https://spack.readthedocs.io/en/v0.19.1/environments.html#spec-concretization

fixes #1365

[vsoch]

It looks like the automated build was disabled - @hainest you might want to test this locally to make sure the new spack builds (can't assume that it does!)

[hainest]

@weixiao-zhan How are you building Dyninst that you ran into this? I disabled these builds because I was tired of chasing spack and our rate of change was small enough that local builds were fine.

[weixiao-zhan]

Yes, I was rebuilding Dyninst from the latest master branch (I was previously using v12.1.0, and I wanted to make sure the ENDBR patch works with the latest code). As mentioned in issue #1365 , I changed the yaml following their doc. Before spack changes their commands again, this should work.

[hainest]

Ok. If it's working for you, then it's fine to merge. If you could 'Squash and Merge' that would be great.