fix(github): drop redundant is:pr from gh pr list search (#34633)

gh pr list already scopes --search with repo:OWNER/REPO type:pr. The
extra is:pr Orca added made private-repo PR lists come back empty when
gh is authenticated with a GitHub App user token (ghu_).

Fixes #34536

src/main/github/client-issue-source.test.ts

@@ -113,7 +113,7 @@ function issueSearchArgs(
   ]
 }
 
-function prListArgs(ownerRepo: string, query = 'is:pr is:open'): string[] {
+function prListArgs(ownerRepo: string, query = 'is:open'): string[] {
   return [
     'pr',
     'list',