chore: allow manual cache update in GH actions (#1118)

sort of testing a hypothesis, but i suspect the build cache needs to be
rebuilt against [this](#1117)
branch in order for
[this](https://github.com/TBD54566975/ftl/actions/runs/8370343139/job/22939654513)
to pass

if this fixes the problem, we should probably figure out a better
approach to ensuring the kotlin-runtime JAR is up-to-date with the local
Kotlin when running Go tests, now that Go tests also exercise Kotlin
code

.github/workflows/writecache.yml

@@ -2,6 +2,7 @@ on:
   push:
     branches:
       - main
+  workflow_dispatch:
 concurrency:
   group: ${{ github.ref }}-writecache
   cancel-in-progress: true