ci(zizmor): trigger on push to master, not the non-existent main (#1707)

The default branch is master; there is no main branch, so the push-triggered
zizmor scan never runs on the default branch (only PRs are covered). Point it
at master so the push scan and code-scanning baseline actually populate.

Co-authored-by: Jordan Bayles <bayles.jordan@gmail.com>

.github/workflows/zizmor.yml

diff --git a/.github/workflows/zizmor.yml b/.github/workflows/zizmor.yml
index b0f811f..eebbaa2 100644
--- a/.github/workflows/zizmor.yml
+++ b/.github/workflows/zizmor.yml
@@ -2,7 +2,7 @@
 
 on:
   push:
-    branches: ["main"]
+    branches: ["master"]
   pull_request:
     branches: ["**"]