Search $PREFIX/share/locale for translations on Unix

The changes introduced in ​ea160f0 accidentally omitted this search
location, so only wx's own install prefix was searched. This probably
went undetected for so long because these two are often, but not always,
the same.

Fixes #17740.

(cherry picked from commit 2c24ee9216)

src/common/translation.cpp

@@ -1812,6 +1812,12 @@ wxArrayString GetSearchPrefixes()
     stdp = wxStandardPaths::Get().GetResourcesDir();
     if ( paths.Index(stdp) == wxNOT_FOUND )
         paths.Add(stdp);
+
+  #if defined(__UNIX__) && !defined(__WXOSX_COCOA__) && !defined(__WXOSX_IPHONE__) && !defined(__DARWIN__)
+    stdp = wxStandardPaths::Get().GetInstallPrefix() + "/share/locale";
+    if ( paths.Index(stdp) == wxNOT_FOUND )
+        paths.Add(stdp);
+  #endif
 #endif // wxUSE_STDPATHS
 
     // last look in default locations