linux/development_tools: add undocumented environment variables.

This makes it easier to test the automatic installation of the libc and
compiler formulae without having to change the code.

This is particularly useful now we don't have any official Docker images
for this.

Author: MikeMcQuaid

Library/Homebrew/extend/os/linux/development_tools.rb

@@ -34,8 +34,12 @@ def default_compiler = :gcc
         def needs_libc_formula?
           return @needs_libc_formula unless @needs_libc_formula.nil?
 
-          @needs_libc_formula = T.let(OS::Linux::Glibc.below_ci_version?, T.nilable(T::Boolean))
-          @needs_libc_formula = !!@needs_libc_formula
+          @needs_libc_formula = T.let(nil, T.nilable(T::Boolean))
+
+          # Undocumented environment variable to make it easier to test libc
+          # formula automatic installation.
+          @needs_libc_formula = true if ENV["HOMEBREW_FORCE_LIBC_FORMULA"]
+          @needs_libc_formula ||= OS::Linux::Glibc.below_ci_version?
         end
 
         # Keep this method around for now to make it easier to add this functionality later.
@@ -52,7 +56,12 @@ def needs_compiler_formula?
           return @needs_compiler_formula unless @needs_compiler_formula.nil?
 
           @needs_compiler_formula = T.let(nil, T.nilable(T::Boolean))
-          @needs_compiler_formula = if host_gcc_path.exist?
+
+          # Undocumented environment variable to make it easier to test compiler
+          # formula automatic installation.
+          @needs_compiler_formula = true if ENV["HOMEBREW_FORCE_COMPILER_FORMULA"]
+
+          @needs_compiler_formula ||= if host_gcc_path.exist?
             ::DevelopmentTools.gcc_version(host_gcc_path.to_s) < OS::LINUX_GCC_CI_VERSION
           else
             true