Make git-annex enable-tor work when using the linux standalone build

Clean the standalone environment before running the su command
to run "sh". Otherwise, PATH leaked through, causing it to run
git-annex.linux/bin/sh, but GIT_ANNEX_DIR was not set,
which caused that script to not work:

[2022-10-26 15:07:02.145466106] (Utility.Process) process [938146] call: pkexec ["sh","-c","cd '/home/joey/tmp/git-annex.linux/r' && '/home/joey/tmp/git-annex.linux/git-annex' 'enable-tor' '1000'"]
/home/joey/tmp/git-annex.linux/bin/sh: 4: exec: /exe/sh: not found

Changed programPath to not use GIT_ANNEX_PROGRAMPATH,
but instead run the scripts at the top of GIT_ANNEX_DIR.
That works both when the standalone environment is set up, and when it's
not.

Sponsored-by: Kevin Mueller on Patreon

Command/WebApp.hs

@@ -22,6 +22,7 @@ import Utility.WebApp
 import Utility.Daemon (checkDaemon)
 import Utility.UserInfo
 import Annex.Init
+import Annex.Path
 import qualified Git
 import Git.Types (fromConfigValue)
 import qualified Git.Config
@@ -222,7 +223,7 @@ openBrowser' mcmd htmlshim realurl outh errh =
 #endif
 		hPutStrLn (fromMaybe stdout outh) $ "Launching web browser on " ++ url
 		hFlush stdout
-		environ <- cleanEnvironment
+		environ <- cleanStandaloneEnvironment
 		let p' = p
 			{ env = environ
 			, std_out = maybe Inherit UseHandle outh