```ci: standardize github workflow to use lowercase loader names

Update GitHub Actions workflows to specify 'forge' and 'fabric' loaders in lowercase
for consistency and to avoid potential case-sensitive issues on different operating
environments. Affected workflows include 'build.yml' and 'build-pr.yml'.
```

.github/workflows/build-pr.yml

@@ -20,7 +20,7 @@ jobs:
     runs-on: windows-latest
     strategy:
       matrix:
-        loader: [ 'Forge', 'Fabric' ]
+        loader: [ 'forge', 'fabric' ]
 
     steps:
       - name: 'Checkout repository'

.github/workflows/build.yml

@@ -16,7 +16,7 @@ jobs:
     runs-on: windows-latest
     strategy:
       matrix:
-        loader: [ 'Forge', 'Fabric' ]
+        loader: [ 'forge', 'fabric' ]
 
     steps:
       - name: 'Checkout repository'