sudo_gui_script_contents: add timeout, remove unnecessaries

[toobuntu]

First commit

• Adds a timeout to pinentry-mac. This way, if Homebrew Autoupdate attempts an upgrade with --sudo when the user is not available to enter the admin password, the upgrade of that cask can fail and any remaining upgrades can proceed.

Second commit

• Eliminates the need to store the admin password in a variable and then print that var to stdout. sudo --askpass expects the helper program to output to stdout. pinentry already prints to stdout. That's processed with awk and also printed to stdout already. Addresses my comment in issue #40.
• While there, limit the scope of PATH. We already require the formula pinentry-mac, which is linked to HOMEBREW_PREFIX/bin. So, set that as PATH and use a full path for /usr/bin/awk. That way, we are (more) sure what awk is going to do to the password, too. (An alternative to setting PATH altogether would be calling pinentry like /usr/bin/env -P"$HOMEBREW_PREFIX/bin" pinentry-mac.)
• Just set PATH. There is no need to export it.

[toobuntu]

Verified that the askpass helper does get executed without exporting PATH:

# success
sudo --reset-timestamp; env -i HOMEBREW_PREFIX="$HOMEBREW_PREFIX" SUDO_ASKPASS="$HOME/devel/sudo_gui_script_contents" sudo --validate --askpass

# pinentry-mac: command not found
sudo --reset-timestamp; env -i SUDO_ASKPASS="$HOME/devel/sudo_gui_script_contents" sudo --validate --askpass

[DomT4]

Makes sense to me. Thank you @toobuntu and @swissbuechi for the review also.