[Github] Add repository checks to release-binaries workflow (#84437)

This patch adds repository checks to the release-binaries workflow jobs.
People were observing that the job was running on a schedule in their
forks. This only happens on old forks, but those probably exist in great
number given how prolific LLVM is. This is also good practice anyways,
on top of solving the direct problem of these jobs running with the cron
schedule on people's forks.

Author: boomanaiden154

.github/workflows/release-binaries.yml

@@ -35,6 +35,7 @@ jobs:
   prepare:
     name: Prepare to build binaries
     runs-on: ubuntu-22.04
+    if: github.repository == 'llvm/llvm-project'
     outputs:
       release-version: ${{ steps.vars.outputs.release-version }}
       flags: ${{ steps.vars.outputs.flags }}
@@ -85,6 +86,7 @@ jobs:
     name: "Fill Cache ${{ matrix.os }}"
     needs: prepare
     runs-on: ${{ matrix.os }}
+    if: github.repository == 'llvm/llvm-project'
     strategy:
       matrix:
         os:
@@ -119,6 +121,7 @@ jobs:
       - prepare
       - fill-cache
     runs-on: ${{ matrix.target.runs-on }}
+    if: github.repository == 'llvm/llvm-project'
     strategy:
       fail-fast: false
       matrix: