Replace environment variables in PATH prefix

The PATH prefix modified through the GUI was set without
replacing embedded environment variables. This might have
caused problems on Windows with external python installations.

(cherry picked from commit 472bdf5910)

src/LyXRC.cpp

@@ -2997,7 +2997,7 @@ void actOnUpdatedPrefs(LyXRC const & lyxrc_orig, LyXRC const & lyxrc_new)
 	case LyXRC::RC_PARAGRAPH_MARKERS:
 	case LyXRC::RC_PATH_PREFIX:
 		if (lyxrc_orig.path_prefix != lyxrc_new.path_prefix) {
-			prependEnvPath("PATH", lyxrc_new.path_prefix);
+			prependEnvPath("PATH", replaceEnvironmentPath(lyxrc_new.path_prefix));
 			// Resets python path
 			support::os::python(true);
 		}

status.23x

@@ -136,6 +136,8 @@ What's new
 
 - Fix handling of labels with change tracking (bug 6563).
 
+- Expand environment variables when modifying the PATH prefix.
+
 
 
 * INTERNALS