Output version string in build workflow.

This commit makes the build workflow output the version string as found
in the `version` property in `build.gradle.kts`. The version string will
be necessary further down the pipeline when we need to extract release
notes and create tags.

There are many ways to extract the version string:

- Use `grep` to grab it from `build.gradle.kts` directly. This is pretty
  brittle, since we don't really know for sure if the structure of the
  file will change in the future.

- Create a Gradle task in `build.gradle.kts` that prints the version
  string. This is probably the most direct approach we could take, but
  such a hyper-specific task feels like a code smell. It also requires
  running Gradle again, which is a bit slow.

- Use the built-in `properties` task in Gradle to print the `version`
  property and `grep` it. We avoid changing `build.gradle.kts`, which is
  a plus, but we still have to actually run Gradle.

- Parse the filename of the resulting jar-file in `build/libs/`, since
  it now contains the version string. This is also brittle, because we
  don't know if we're gonna continue to append the version string to the
  jar-file, and depending so much on it being there is a little scary.

- Extract `plugin.yml` from the resulting jar-file and `grep` it. This
  is perhaps a little crude, but it is much faster than running Gradle,
  and as a bonus, we get a bit closer to "what's inside the jar-file",
  which should give us a bit more confidence that any given release is
  actually the version it claims to be.

It may seem like a small thing to invest so much text on, but from an
automation standpoint, it is much easier to be confident in automations
with predictable and robust mechanisms for deriving input variables.

.github/workflows/build.yml

@@ -10,6 +10,9 @@ jobs:
   build:
     runs-on: ubuntu-latest
 
+    outputs:
+      version: ${{ steps.version.outputs.version }}
+
     steps:
       - name: 'Checkout'
         uses: actions/checkout@v4
@@ -29,3 +32,14 @@ jobs:
         with:
           name: MobArena.jar
           path: build/libs/MobArena-*.jar
+
+      - name: 'Output version'
+        id: version
+        run: |
+          version=$(
+            unzip -p build/libs/MobArena-*.jar plugin.yml \
+            | grep '^version: ' \
+            | awk '{printf $2}' \
+            | tr -d "'" \
+          )
+          echo "version=${version}" >> "${GITHUB_OUTPUT}"