Fix invalid GitHub Action YAML files (agama-project#1707)

## Problem

- The GitHub Actions fail with this error: `The workflow is not valid.
.github/workflows/weblate-update-pot.yml: Unexpected tag
'!github.event.repository.fork'`

## Solution

- From the documentation: You must always use the `${{ }}` expression
syntax or escape with `''`, `""`, or `()` when the expression starts
with `!`, since `!` is reserved notation in YAML format.
- See
https://docs.github.com/en/actions/writing-workflows/choosing-when-your-workflow-runs/using-conditions-to-control-job-execution

.github/workflows/weblate-merge-po.yml

@@ -16,7 +16,7 @@ jobs:
       pull-requests: write
 
     # do not run in forks
-    if: !github.event.repository.fork
+    if: ${{ !github.event.repository.fork }}
 
     runs-on: ubuntu-latest
 

.github/workflows/weblate-merge-products-po.yml

@@ -16,7 +16,7 @@ jobs:
       pull-requests: write
 
     # do not run in forks
-    if: !github.event.repository.fork
+    if: ${{ !github.event.repository.fork }}
 
     runs-on: ubuntu-latest
 

.github/workflows/weblate-merge-service-po.yml

@@ -16,7 +16,7 @@ jobs:
       pull-requests: write
 
     # do not run in forks
-    if: !github.event.repository.fork
+    if: ${{ !github.event.repository.fork }}
 
     runs-on: ubuntu-latest
 

.github/workflows/weblate-update-pot.yml

@@ -11,7 +11,7 @@ on:
 jobs:
   update-pot:
     # do not run in forks
-    if: !github.event.repository.fork
+    if: ${{ !github.event.repository.fork }}
 
     runs-on: ubuntu-latest