formula_installer.rb: Hotfix for head installs

For now, don't strip `--HEAD` from `ARGV` prior to forking for install.
Currently this doesn't work because the formula object the installer is acting
on has its `prefix` set while under the influence of `--HEAD`. So, the
installer bails out when it sees that nothing is installed to `f.prefix`.

This is a hotfix for issue Homebrew/homebrew#8668 to allow formulae to keep building. A proper
fix should ensure that formula installers are operating on objects with
appropriately set `prefix` values.

Library/Homebrew/formula_installer.rb

@@ -253,7 +253,7 @@ class FormulaInstaller
       # list.
       #
       # FIXME:
-      # `ARGV.formulae` probably should be throwing exceptions, it should be
+      # `ARGV.formulae` probably shouldn't be throwing exceptions, it should be
       # the caller's responsibility to check `ARGV.formulae.empty?`.
       return false if ARGV.named.empty?
       ARGV.formulae.include? f
@@ -261,10 +261,14 @@ class FormulaInstaller
 
     args = ARGV.clone
 
+    # FIXME: Also need to remove `--HEAD`, however there is a problem in that
+    # the properties of formula objects, such as `prefix`, are influenced by
+    # `--HEAD` and get set when the object is created.
+    #
+    # See issue #8668
     %w[
       --debug -d
       --fresh
-      --HEAD
       --interactive -i
       --verbose -v
     ].each {|flag| args.delete flag} unless explicitly_requested?