language/*: fix *_SHEBANG_REGEX anchors

We should use `\A` to pick up shebangs at the start of files instead of
just anywhere in a given script.

See Homebrew/homebrew-core#227654 for an example where this can cause
problems.

Library/Homebrew/language/node.rb

@@ -96,7 +96,7 @@ module Language
       module_function
 
       # A regex to match potential shebang permutations.
-      NODE_SHEBANG_REGEX = %r{^#! ?(?:/usr/bin/(?:env )?)?node( |$)}
+      NODE_SHEBANG_REGEX = %r{\A#! ?(?:/usr/bin/(?:env )?)?node( |$)}
 
       # The length of the longest shebang matching `SHEBANG_REGEX`.
       NODE_SHEBANG_MAX_LENGTH = T.let("#! /usr/bin/env node ".length, Integer)

Library/Homebrew/language/perl.rb

@@ -15,7 +15,7 @@ module Language
       module_function
 
       # A regex to match potential shebang permutations.
-      PERL_SHEBANG_REGEX = %r{^#! ?(?:/usr/bin/(?:env )?)?perl( |$)}
+      PERL_SHEBANG_REGEX = %r{\A#! ?(?:/usr/bin/(?:env )?)?perl( |$)}
 
       # The length of the longest shebang matching `SHEBANG_REGEX`.
       PERL_SHEBANG_MAX_LENGTH = T.let("#! /usr/bin/env perl ".length, Integer)

Library/Homebrew/language/php.rb

@@ -15,7 +15,7 @@ module Language
       module_function
 
       # A regex to match potential shebang permutations.
-      PHP_SHEBANG_REGEX = %r{^#! ?(?:/usr/bin/(?:env )?)?php( |$)}
+      PHP_SHEBANG_REGEX = %r{\A#! ?(?:/usr/bin/(?:env )?)?php( |$)}
 
       # The length of the longest shebang matching `SHEBANG_REGEX`.
       PHP_SHEBANG_MAX_LENGTH = T.let("#! /usr/bin/env php ".length, Integer)

Library/Homebrew/language/python.rb

@@ -112,7 +112,7 @@ module Language
       module_function
 
       # A regex to match potential shebang permutations.
-      PYTHON_SHEBANG_REGEX = %r{^#! ?(?:/usr/bin/(?:env )?)?python(?:[23](?:\.\d{1,2})?)?( |$)}
+      PYTHON_SHEBANG_REGEX = %r{\A#! ?(?:/usr/bin/(?:env )?)?python(?:[23](?:\.\d{1,2})?)?( |$)}
 
       # The length of the longest shebang matching `SHEBANG_REGEX`.
       PYTHON_SHEBANG_MAX_LENGTH = T.let("#! /usr/bin/env pythonx.yyy ".length, Integer)