Transform exit handlers of the form
trap cleanup EXIT [INT] [TERM] [QUIT] [HUP] [ERR]
(where cleanup is idempotent)
to
trap cleanup EXIT
This fixes a common bash antipattern.
Each of the above signals causes the script to exit. For each signal,
bash first handles the signal by running `cleanup` and then runs
`cleanup` again when handling EXIT.
(Exception: `vscode/*` prevents the second run of `cleanup` by removing
the trap in cleanup`).
Simplify the cleanup logic by just trapping exit, which is always run
when the script exits due to any of the above signals.
Note: In case of borgbackup, the exit handler is not idempotent, but just
trapping EXIT guarantees that it's only run once.
This simplifies the following commit.
- Remove `--` from `rm` because `$tmpDir` is guaranteed to never start
with a `-`.
We also don't use this pattern when removing tmpDirs at other places
in nixpkgs.
- Remove `unset tmpDir` because the variable `tmpDir` is never
accessed after the exit handler is run.
