pressure-vessel: Remove the concept of "locking" environment variables

This was only necessary because we were reusing a single container across multiple entry-point invocations, and expecting "most" arbitrary environment variables from each new invocation to be taken into account for commands running in the container, which meant that we needed to keep track of which environment variables had to be exceptions to that rule for technical reasons. Now that we're no longer injecting multiple commands into the same container like that, we don't need this complexity.

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Ready for review, but cannot be merged until steamlinuxruntime!53 (merged) lands.