commands/exec: allow customising environment.

Provide `HOMEBREW_BUNDLE_EXEC_ALL_KEG_ONLY_DEPS` and
`HOMEBREW_BUNDLE_EXEC_FORMULA_VERSION_*` variables to allow adjusting
the environment variables that are set in `brew bundle exec`.

lib/bundle/commands/exec.rb

@@ -35,7 +35,15 @@ def run(*args, global: false, file: nil)
 
           Formulary.factory(entry.name)
         end
-        ENV.keg_only_deps = ENV.deps.select(&:keg_only?)
+
+        # Allow setting all dependencies to be keg-only
+        # (i.e. should be explicitly in HOMEBREW_*PATHs ahead of HOMEBREW_PREFIX)
+        ENV.keg_only_deps = if ENV["HOMEBREW_BUNDLE_EXEC_ALL_KEG_ONLY_DEPS"].present?
+          ENV.delete("HOMEBREW_BUNDLE_EXEC_ALL_KEG_ONLY_DEPS")
+          ENV.deps
+        else
+          ENV.deps.select(&:keg_only?)
+        end
         ENV.setup_build_environment
 
         # Enable compiler flag filtering
@@ -58,6 +66,27 @@ def run(*args, global: false, file: nil)
         # Ensure the Ruby path we saved goes before anything else, if the command was in the PATH
         ENV.prepend_path "PATH", command_path if command_path.present?
 
+        # Replace the formula versions from the environment variables
+        formula_versions = {}
+        ENV.each do |key, value|
+          match = key.match(/^HOMEBREW_BUNDLE_EXEC_FORMULA_VERSION_(.+)$/)
+          next if match.blank?
+
+          formula_name = match[1]
+          next if formula_name.blank?
+
+          ENV.delete(key)
+          formula_versions[formula_name.downcase] = value
+        end
+        formula_versions.each do |formula_name, formula_version|
+          ENV.each do |key, value|
+            opt = %r{/opt/#{formula_name}([/:$])}
+            next unless value.match(opt)
+
+            ENV[key] = value.gsub(opt, "/Cellar/#{formula_name}/#{formula_version}\\1")
+          end
+        end
+
         exec(*args)
       end
     end