Previously, bootstrap was only installed if `$PREFIX` didn't exist, was empty or only had `$PREFIX/tmp`. But now with 03e1d14e, `$PREFIX/etc/termux/termux.env` was also created at app startup before bootstrap check was made, hence it was being assumed that bootstrap was already installed.
Now, bootstrap will be installed even if `$PREFIX/tmp`, `$PREFIX/etc/termux/termux.env.tmp` or `$PREFIX/etc/termux/termux.env` exist but no other files do.
Closes#2844
a2df7d791a