fix(yarn): don't base64 encode npmAuthIdent (#11873)

lib/manager/npm/post-update/__snapshots__/rules.spec.ts.snap

@@ -11,7 +11,7 @@ Object {
   "additionalYarnRcYml": Object {
     "npmRegistries": Object {
       "//registry.company.com/": Object {
-        "npmAuthIdent": "dXNlcjEyMzpwYXNzMTIz",
+        "npmAuthIdent": "user123:pass123",
       },
       "//registry.npmjs.org": Object {
         "npmAuthToken": "token123",
@@ -33,7 +33,7 @@ Object {
   "additionalYarnRcYml": Object {
     "npmRegistries": Object {
       "//registry.company.com/": Object {
-        "npmAuthIdent": "dXNlcjEyMzpwYXNzMTIz",
+        "npmAuthIdent": "user123:pass123",
       },
     },
   },

lib/manager/npm/post-update/rules.ts

@@ -38,9 +38,7 @@ export function processHostRules(): HostRulesResult {
         additionalNpmrcContent.push(`${uri}:_password=${password}`);
         additionalYarnRcYml ||= { npmRegistries: {} };
         additionalYarnRcYml.npmRegistries[uri] = {
-          npmAuthIdent: Buffer.from(
-            `${hostRule.username}:${hostRule.password}`
-          ).toString('base64'),
+          npmAuthIdent: `${hostRule.username}:${hostRule.password}`,
         };
       }
     }