Gradle build task improvements (#2164)

## Description

This fixes a few problems with the Gradle `build` task and subtasks.
1. Spotless was being run on `node_modules`, resulting in errors for
out-of-source files. This is now disabled.
2. All tests were running from the `build` task, resulting in unexpected
windows popping up. Now only headless tests are run.
3. Headless tests were updated to run from the same root directory as
the other tests.

## Meta

Merge checklist:
- [X] Pull Request title is [short, imperative
summary](https://cbea.ms/git-commit/) of proposed changes
- [X] The description documents the _what_ and _why_
- [ ] If this PR changes behavior or adds a feature, user documentation
is updated
- [ ] If this PR touches photon-serde, all messages have been
regenerated and hashes have not changed unexpectedly
- [ ] If this PR touches configuration, this is backwards compatible
with settings back to v2025.3.2
- [ ] If this PR touches pipeline settings or anything related to data
exchange, the frontend typing is updated
- [ ] If this PR addresses a bug, a regression test for it is added

shared/common.gradle

@@ -71,7 +71,7 @@ tasks.register('testHeadless', Test) {
         showStandardStreams = true
     }
     exclude '**/*BenchmarkTest*'
-    workingDir = "../"
+    workingDir = new File("${rootDir}")
 }
 
 jacoco {
@@ -96,3 +96,10 @@ jacocoTestReport {
         }))
     }
 }
+
+// Only run headless tests from the build task
+gradle.taskGraph.whenReady { graph ->
+    if (graph.hasTask(build)) {
+        test.enabled = false
+    }
+}