Commit sha in solutions_sha.txt is wrong after squash-and-merge PRs

[bernhardpg]

For a PR to manipulation-solutions that uses squash and merge, the single commit that is commited to master will have a brand new sha value (it won't be any of the sha's in the PRs commit history). Therefore, the workflow in README.md in the solutions repo is wrong (one has to first merge to manipulation-solutions, not the other way around as the README suggests).

A current solution is:

1. Merge the solutions to master in the manipulation-solutions repo
2. In the PR in the manipulation repo: Run solutions/install.py to generate a solutions_sha.txt with the correct commit sha
3. CI/tests will now run in the PR in the manipulation repo for the newly commited solutions.

The problem with this is that if the tests fail, then one has to go back and update the manipulation-solutions repo again. But I don't really know if there is a way to avoid this.