emacs-30: allow to pin revision using env var (#653)

Formula/emacs-plus@30.rb

@@ -77,7 +77,11 @@ class EmacsPlusAT30 < EmacsBase
   # URL
   #
 
-  url "https://github.com/emacs-mirror/emacs.git", :branch => "master"
+  if ENV['HOMEBREW_EMACS_PLUS_30_REVISION']
+    url "https://github.com/emacs-mirror/emacs.git", :revision => ENV['HOMEBREW_EMACS_PLUS_30_REVISION']
+  else
+    url "https://github.com/emacs-mirror/emacs.git", :branch => "master"
+  end
 
   #
   # Icons

README.org

@@ -430,23 +430,14 @@ $ brew install cask --ignore-dependencies
 
 The most current development version is consistently compiled from a specific branch. At the time of this writing, for instance, =emacs-plus@30= is being built from the =master= branch.
 
-Here's the relevant code snippet for the =emacs-plus@30= formula:
+Ordinarily, =brew= will update to the latest commit in the specified branch during installation, meaning the exact commit you're building from varies depending on the installation timing. As development versions are less stable than official releases, some users may prefer to pin a specific commit.
 
-#+begin_src ruby
-    # Formula/[email protected]
-    url "https://github.com/emacs-mirror/emacs.git", :branch => "master"
-#+end_src
-
-Ordinarily, =brew= will update to the latest commit in the specified branch during installation, meaning the exact commit you're building from varies depending on the installation timing. As development versions are less stable than official releases, some users may prefer to pin a specific commit. To do this, one must manually modify the formula by executing =brew edit emacs-plus@30= and altering the =url= directive to reference a specific commit, as shown below:
+Usually this is done by modifying formula (i.e. by executing =brew edit emacs-plus@30= and altering the =url= directive), but you can also set =HOMEBREW_EMACS_PLUS_30_REVISION= environment variable before installing =emacs-plus@30=:
 
-#+begin_src ruby
-    url "https://github.com/emacs-mirror/emacs.git", :revision => "6abea4d98d1d964c68a78cb9b5321071da851654"
+#+begin_src bash
+  $ HOMEBREW_EMACS_PLUS_30_REVISION=6abea4d98d1d964c68a78cb9b5321071da851654 brew install emacs-plus@30 [OPTIONS]
 #+end_src
 
-#+begin_quote
-⚠️ It's important to note that any local changes to commits or branches within the cached repository won't impact the build process, as =brew= overrides the state of the cloned repository during installation.
-#+end_quote
-
 ** Screenshots
 
 #+BEGIN_HTML