ci(gosec): use go.mod for Go version and add manual trigger

The scheduled Gosec run silently produced 0 findings because setup-go
resolved 'go-version: 1.26.x' to an older patch than go.mod requires
(go 1.26.5) while GOTOOLCHAIN=local blocked a toolchain download, so the
package loader failed for every package.

Read the Go version from go.mod so it can never drift from the module's
requirement, and add workflow_dispatch for manual runs.

Author: libotony

.github/workflows/gosec.yaml

@@ -1,5 +1,6 @@
 name: Gosec
 on:
+  workflow_dispatch:
   schedule:
   #   # This is meant to run every day at 8am
     - cron:  '0 8 * * 1-5'
@@ -19,7 +20,7 @@ jobs:
         uses: actions/checkout@v5
       - uses: actions/setup-go@v5
         with:
-          go-version: '1.26.x'
+          go-version-file: 'go.mod'
           cache: false
       - name: Run Gosec
         id: gosec-run