Don’t use `execvp` when we know the path
(cherry picked from commit dfa219d03b2277e61dde1c1ddddddff7411ef112)
Change-Id: I2226340cebbe935e1f2fe10207daba69683c8cb3
These methods would previously fail on the other `Installable`s, so
moving them to this class is more correct as to where they actually
work.
Additionally, a `InstallableValueCommand` is created to make it easier
(or rather no worse than before) to write commands that just work on
`InstallableValue`s.
Besides being a cleanup to avoid failing default methods, this gets us
closer to https://github.com/NixOS/rfcs/pull/134.
eldritch horrors