Add caching to CLT installation check

[willmcginnis]

Improve the should_install_command_line_tools function by:

1. Caching the result in SHOULD_INSTALL_CLT to avoid redundant file existence checks.
2. Using a local variable (ret) to capture return values for added clarity.

This change slightly increases code size but provides a more efficient
and readable approach.

Upsides:

• Efficiency improvement by skipping redundant checks on subsequent calls.

Downsides:

• Introduces a new global variable (SHOULD_INSTALL_CLT).
• Increases the function length by about 10 lines.

[gromgit]

The backslash is not needed.

[gromgit]

I'm not sure calling should_install_command_line_tools is computationally expensive enough to warrant caching, but even if it is, the required change can be reduced to just 3 lines (1 new, 2 changed):

diff --git a/install.sh b/install.sh
index 71ca08b..4b588fa 100755
--- a/install.sh
+++ b/install.sh
@@ -732,6 +732,7 @@ fi
 if should_install_command_line_tools
 then
   ohai "The Xcode Command Line Tools will be installed."
+  SHOULD_INSTALL_CLT=1
 fi
 
 non_default_repos=""
@@ -834,7 +835,7 @@ then
   execute "${TOUCH[@]}" "${HOMEBREW_CACHE}/.cleaned"
 fi
 
-if should_install_command_line_tools && version_ge "${macos_version}" "10.13"
+if [[ -n "${SHOULD_INSTALL_CLT-}" ]] && version_ge "${macos_version}" "10.13"
 then
   ohai "Searching online for the Command Line Tools"
   # This temporary file prompts the 'softwareupdate' utility to list the Command Line Tools
@@ -859,7 +860,7 @@ then
 fi
 
 # Headless install may have failed, so fallback to original 'xcode-select' method
-if should_install_command_line_tools && test -t 0
+if [[ -n "${SHOULD_INSTALL_CLT-}" ]] && test -t 0
 then
   ohai "Installing the Command Line Tools (expect a GUI popup):"
   execute "/usr/bin/xcode-select" "--install"

[MikeMcQuaid]

Passing on this, sorry. As @gromgit mentioned: this is not expensive enough to warrant caching. Also, there's a non-zero chance at install time that these files are present at some points and not at others.