This improves the load time of most brew commands. For an example of
one of the simplest commands this speeds up:
Without Bootsnap:
```
$ hyperfine 'git checkout master; brew help' 'git checkout optimise_requires; brew help'
Benchmark 1: git checkout master; brew help
Time (mean ± σ): 525.0 ms ± 35.8 ms [User: 229.9 ms, System: 113.1 ms]
Range (min … max): 465.3 ms … 576.6 ms 10 runs
Benchmark 2: git checkout optimise_requires; brew help
Time (mean ± σ): 383.3 ms ± 25.1 ms [User: 133.0 ms, System: 72.1 ms]
Range (min … max): 353.0 ms … 443.6 ms 10 runs
Summary
git checkout optimise_requires; brew help ran
1.37 ± 0.13 times faster than git checkout master; brew help
```
With Bootsnap:
```
$ hyperfine 'git checkout master; brew help' 'git checkout optimise_requires; brew help'
Benchmark 1: git checkout master; brew help
Time (mean ± σ): 386.0 ms ± 30.9 ms [User: 130.2 ms, System: 93.8 ms]
Range (min … max): 359.5 ms … 469.3 ms 10 runs
Benchmark 2: git checkout optimise_requires; brew help
Time (mean ± σ): 330.2 ms ± 32.4 ms [User: 93.4 ms, System: 73.0 ms]
Range (min … max): 302.9 ms … 413.9 ms 10 runs
Summary
git checkout optimise_requires; brew help ran
1.17 ± 0.15 times faster than git checkout master; brew help
```
This environment variable allows telling Homebrew to use the `SUDO_USER`
variable to `sudo` through that user when Homebrew (Cask) attempts to
run `sudo`.
While we're here, clarify in some messaging that we're running `sudo`
and that that's the password we're asking for; the specific password is
configuration dependent and not the specific password for the user.
Similarly, remove the `Package installers may write to any location`
output; it's kinda spammy and doesn't feel like the right place.
With sudoers one may override default sudo user. This mostly works
provided the admin configured the replacement appropriately. However
there are exceptions that absolutely must be run by root such as
/usr/sbin/installer and, under certain circumstances, /bin/launchctl.
Using /usr/bin/env as a frontend for the actual command
prevents sudoers from restricting allowed commands and configuring
detailed command environment.
