Set TMPDIR for Xcode’s make

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>

Library/Homebrew/utils/gems.rb

@@ -44,6 +44,10 @@ module Homebrew
     ENV["GEM_HOME"] = gem_home
     ENV["GEM_PATH"] = ENV["GEM_HOME"]
 
+    # Set TMPDIR so Xcode's `make` doesn't fall back to `/var/tmp/`,
+    # which may be not user-writable.
+    ENV["TMPDIR"] = ENV["HOMEBREW_TEMP"]
+
     # Make RubyGems notice environment changes.
     require "rubygems"
     Gem.clear_paths