Bug 1652068: Log mozbuild prioritization on configure path r=firefox-build-system-reviewers,rstewart

The order in which toolchain binaries are resolved change based on environmental factors,
such as whether Firefox is being built in release mode or not.

An informative log was added in either case.

Differential Revision: https://phabricator.services.mozilla.com/D84439

build/moz.configure/toolchain.configure

@@ -688,16 +688,17 @@ def original_path():
 
 @template
 def toolchain_search_path_for(host_or_target):
-    vc_path = {
-        host: host_vc_compiler_path,
-        target: vc_compiler_path,
+    arch_alias, vc_path = {
+        host: ('host', host_vc_compiler_path),
+        target: ('target', vc_compiler_path),
     }[host_or_target]
 
-    @depends(vc_path, original_path, developer_options, mozbuild_state_path)
+    @depends(dependable(arch_alias), vc_path, original_path, developer_options,
+             mozbuild_state_path)
     @imports('os')
     @imports(_from='os', _import='environ')
-    def toolchain_search_path(vc_compiler_path, original_path, developer_options,
-                              mozbuild_state_path):
+    def toolchain_search_path(arch_alias, vc_compiler_path, original_path,
+                              developer_options, mozbuild_state_path):
         result = list(original_path)
 
         if vc_compiler_path:
@@ -725,8 +726,17 @@ def toolchain_search_path_for(host_or_target):
         result.append(rustup_path)
 
         if developer_options:
-            return bootstrapped + result
-        return result + bootstrapped
+            log.debug('Prioritizing mozbuild state dir in {} toolchain path because '
+                      'you are not building in release mode.'.format(arch_alias))
+            search_path = bootstrapped + result
+        else:
+            log.debug('Prioritizing system over mozbuild state dir in {} '
+                      'toolchain path because you are building in '
+                      'release mode.'.format(arch_alias))
+            search_path = result + bootstrapped
+
+        log.debug('Search path for {} toolchain: {}'.format(arch_alias, search_path))
+        return search_path
     return toolchain_search_path