[release/2.13] Bump CURRENT_CANDIDATE_VERSION to 2.13.0 (#8172)

The release/2.13 branch cut bumped CURRENT_NIGHTLY_VERSION to 2.13.0 but
left CURRENT_CANDIDATE_VERSION at 2.12.0. For the test channel,
initialize_globals() sets CURRENT_VERSION = CURRENT_CANDIDATE_VERSION
and the generated matrix exposes it as stable_version, which
smoke_test.py enforces. The test channel now serves the 2.13.0 RC, so
validation fails:

RuntimeError: Torch version mismatch, expected 2.12.0 for channel test.
  But its 2.13.0+cpu

Bump CURRENT_CANDIDATE_VERSION to 2.13.0 so the validation matrix
expects the 2.13.0 release candidate. CURRENT_STABLE_VERSION stays
2.12.0 (the current GA). Complements #8171, which pinned test-infra-ref
to release/2.13 so this constant is read from the release branch.

Authored with the assistance of Claude Code.

Author: atalman

tools/scripts/generate_binary_build_matrix.py

@@ -85,7 +85,7 @@
 
 
 CURRENT_NIGHTLY_VERSION = "2.13.0"
-CURRENT_CANDIDATE_VERSION = "2.12.0"
+CURRENT_CANDIDATE_VERSION = "2.13.0"
 CURRENT_STABLE_VERSION = "2.12.0"
 CURRENT_VERSION = CURRENT_STABLE_VERSION