Use nightly tag for legacy x86-64 builder images

shared-docker is switching the legacy LibreSSL 3.9.2 and OpenSSL 1.1.1w
builder images to push nightly builds as "nightly" instead of "latest",
aligning with the convention all the newer multi-arch builders already use.

Author: SeanTAllen

.github/workflows/breakage-against-linux-ponyc-latest.yml

@@ -34,7 +34,7 @@ jobs:
     name: LibreSSL 3.x with ponyc main
     runs-on: ubuntu-latest
     container:
-      image: ghcr.io/ponylang/shared-docker-ci-x86-64-unknown-linux-builder-with-libressl-3.9.2:latest
+      image: ghcr.io/ponylang/shared-docker-ci-x86-64-unknown-linux-builder-with-libressl-3.9.2:nightly
     steps:
       - uses: actions/checkout@v6.0.2
       - name: Test
@@ -76,7 +76,7 @@ jobs:
     name: OpenSSL 1.x with ponyc main
     runs-on: ubuntu-latest
     container:
-      image: ghcr.io/ponylang/shared-docker-ci-x86-64-unknown-linux-builder-with-openssl_1.1.1w:latest
+      image: ghcr.io/ponylang/shared-docker-ci-x86-64-unknown-linux-builder-with-openssl_1.1.1w:nightly
     steps:
       - uses: actions/checkout@v6.0.2
       - name: Test

.github/workflows/pr.yml

@@ -31,7 +31,7 @@ jobs:
     name: LibreSSL 3.x with most recent ponyc release
     runs-on: ubuntu-latest
     container:
-      image: ghcr.io/ponylang/shared-docker-ci-x86-64-unknown-linux-builder-with-libressl-3.9.2:latest
+      image: ghcr.io/ponylang/shared-docker-ci-x86-64-unknown-linux-builder-with-libressl-3.9.2:nightly
     steps:
       - uses: actions/checkout@v6.0.2
       - name: Test
@@ -51,7 +51,7 @@ jobs:
     name: OpenSSL 1.x with most recent ponyc release
     runs-on: ubuntu-latest
     container:
-      image: ghcr.io/ponylang/shared-docker-ci-x86-64-unknown-linux-builder-with-openssl_1.1.1w:latest
+      image: ghcr.io/ponylang/shared-docker-ci-x86-64-unknown-linux-builder-with-openssl_1.1.1w:nightly
     steps:
       - uses: actions/checkout@v6.0.2
       - name: Test