Override runtime by setting $KAKOUNE_RUNTIME

doc/pages/expansions.asciidoc

@@ -306,8 +306,8 @@ The following expansions are supported (with required context _in italics_):
     current register when the mapping was triggered
 
 *%val{runtime}*::
-    directory containing the kak support files, determined from Kakoune's
-    binary location
+    the directory containing the kak support files, which is determined from
+    Kakoune's binary location if `$KAKOUNE_RUNTIME` is not set
 
 *%val{select_mode}*::
     _for commands executed from the object menu's `<a-;>` only_ +

src/main.cc

@@ -138,6 +138,9 @@ inline void write_stderr(StringView str) { try { write(STDERR_FILENO, str); } ca
 
 String runtime_directory()
 {
+    if (const char* runtime_directory = getenv("KAKOUNE_RUNTIME"))
+        return runtime_directory;
+
     char relpath[PATH_MAX+1];
     format_to(relpath, "{}../share/kak", split_path(get_kak_binary_path()).first);
     struct stat st;