This fixes an issue where at least in Xcode 11.0, `make` uses
`/var/tmp` as a fallback for temporary files unless `TMPDIR` is set:
```
$ strings "$(xcrun -f make)" | grep -B 3 fopen
TMPDIR
/var/tmp/
GmXXXXXX
fopen (temporary file)
```
Given that Homebrew filtered `TMPDIR`, and the `/var/tmp` directory may
not be writable for non-root users, this would cause Homebrew’s
build environment to error out:
```
$ brew ruby -e 'puts ENV["TMPDIR"]; puts `: | make -f -`'
```
```
Ignoring bigdecimal-2.0.0 because its extensions are not built. Try: gem pristine bigdecimal --version 2.0.0
[…]
Ignoring zlib-1.1.0 because its extensions are not built. Try: gem pristine zlib --version 1.1.0
make: *** fopen (temporary file): Permission denied. Stop.
```
In practice, this would break `brew audit`, `brew style`, and other
commands, which would run `make` to build native gem extensions.
This commit sets `TMPDIR` to `${HOMEBREW_TEMP}` in the gem environment, which
mirrors the behaviour we already have in other places.
We choose `HOMEBREW_TEMP` because that’s user-controlled but also falls
back to `/tmp` in case `TMPDIR` is not set in the user’s environment.
Thanks to Bo Anderson for helping find the bug.
CC: Bo Anderson <mail@boanderson.me>
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