fix: use exec to invoke the stage-2 bootstrap for non-zip case

[chowder]

When the two-stage bootstrap is used, the parent shell process runs python as a child
process, which changes how signals are propagated. Specifically, if a signal is sent
directly to the parent (e.g. kill $parent), the child process (python) won't receive
it and it will appear to be ignored. This is because the parent process is busy waiting
for the child process.

To fix, invoke the python process using exec instead. Because the process is entirely
replaced, signals are sent directly to the replacement. This can't be used for zip files,
though, because they rely on a catching the exit signal to perform cleanup of the extracted
files.

Fixes #2043

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[rickeylev]

LGTM.

#2045 adds the basics for testing this, so we can put a test in after that lands.