Allow triggering builds manually on woodpecker

We need this in case the release build fails for some reason and need to
re-trigger it manually.

.woodpecker/build.yaml

@@ -2,6 +2,7 @@ when:
   - event: pull_request
   - event: push
     branch: main
+  - event: manual
 
 variables:
   - &plugin-sccache-read-only

.woodpecker/e2e.yaml

@@ -2,6 +2,7 @@ when:
   - event: pull_request
   - event: push
     branch: main
+  - event: manual
 
 variables:
   - &plugin-sccache-read-only

.woodpecker/lint.yaml

@@ -2,6 +2,7 @@ when:
   - event: pull_request
   - event: push
     branch: main
+  - event: manual
 
 variables:
   - &plugin-sccache-read-only

.woodpecker/unit-test.yaml

@@ -2,6 +2,7 @@ when:
   - event: pull_request
   - event: push
     branch: main
+  - event: manual
 
 steps:
   unit-tests: