CI: Pin Github Actions to a particular SHA

[triceo]

It is a potential security problem to depend on Github Actions by tag, such as @v4. The repository can be hijacked, and the tag can be overwritten by some malicious version.

We need to review all of our GHA workflows and convert them to use SHAs.
At the same time, let's also try to reduce the number of 3rd-party GHAs from sources other than Github, further reducing our exposure.