brew.sh: cache HOMEBREW_VERSION on disk.

[MikeMcQuaid]

This is a (relatively, for this Bash script) expensive operation that can be trivially cached so: let's do so.

Speedup on my machine:

$ hyperfine "brew --version" "HOMEBREW_VERSION_CACHE=1 brew --version"
Benchmark 1: brew --version
  Time (mean ± σ):     173.4 ms ±   5.1 ms    [User: 28.7 ms, System: 67.0 ms]
  Range (min … max):   167.3 ms … 182.9 ms    15 runs

Benchmark 2: HOMEBREW_VERSION_CACHE=1 brew --version
  Time (mean ± σ):     145.9 ms ±   4.0 ms    [User: 22.4 ms, System: 33.9 ms]
  Range (min … max):   140.0 ms … 154.5 ms    17 runs

Summary
  HOMEBREW_VERSION_CACHE=1 brew --version ran
    1.19 ± 0.05 times faster than brew --version

While we're here:

• improve a comment
• avoid shelling out to further improve performance in more cases

[MikeMcQuaid]

@Bo98 any ideas for a faster command here? Wondering if there's anything else that can be run with xcrun, execute faster in the typical case but still produce the license issue otherwise 🤔

[Bo98]

For the first time execution, most of the slowness is typically xcrun itself due to searching. If you're still seeing the issue on 2nd execution, try clear the license and see if any of these reproduce:

$ find /usr/bin -samefile /usr/bin/clang -exec basename {} \; | sort
DeRez
GetFileInfo
ResMerger
Rez
SetFile
SplitForks
ar
as
asa
bison
c++
c++filt
c89
c99
cc
clang
clang++
clangd
cmpdylib
codesign_allocate
cpp
ctags
ctf_insert
dsymutil
dwarfdump
flex
flex++
g++
gatherheaderdoc
gcc
gcov
git
git-receive-pack
git-shell
git-upload-archive
git-upload-pack
gm4
gnumake
gperf
hdxml2manxml
headerdoc2html
indent
install_name_tool
ld
lex
libtool
lipo
lldb
llvm-g++
llvm-gcc
lorder
m4
make
mig
nm
nmedit
objdump
otool
pagestuff
pip3
python3
ranlib
resolveLinks
rpcgen
segedit
size
sourcekit-lsp
strings
strip
swift
swiftc
unifdef
unifdefall
vtool
xml2man
yacc

(though ideally one that's already likely to be in the cache - or in the cache soon)

There's also defaults read /Library/Preferences/com.apple.dt.Xcode but then you're on a parade of finding the Xcode install to extract the version from.