A few more gametests, update to Gradle 8.12

Okay, listen. I started writing a few more gametests (see #1682), and
then thought I'd do a cheeky Gradle update. However, that broke
vanilla-extract[^1], and also triggered a load of deprecation warnings,
and at that point it was too late to separate the too.

[^1]: 8975ed5a7b

buildSrc/src/main/kotlin/cc/tweaked/gradle/ProcessHelpers.kt

@@ -4,45 +4,10 @@
 
 package cc.tweaked.gradle
 
-import org.codehaus.groovy.runtime.ProcessGroovyMethods
 import org.gradle.api.GradleException
-import java.io.BufferedReader
 import java.io.File
-import java.io.InputStreamReader
-import java.nio.charset.StandardCharsets
 
 internal object ProcessHelpers {
-    fun startProcess(vararg command: String): Process {
-        // Something randomly passes in "GIT_DIR=" as an environment variable which clobbers everything else. Don't
-        // inherit the environment array!
-        return ProcessBuilder()
-            .command(*command)
-            .redirectError(ProcessBuilder.Redirect.INHERIT)
-            .also { it.environment().clear() }
-            .start()
-    }
-
-    fun captureOut(vararg command: String): String {
-        val process = startProcess(*command)
-        process.outputStream.close()
-
-        val result = ProcessGroovyMethods.getText(process)
-        process.waitForOrThrow("Failed to run command")
-        return result
-    }
-
-    fun captureLines(vararg command: String): List<String> {
-        val process = startProcess(*command)
-        process.outputStream.close()
-
-        val out = BufferedReader(InputStreamReader(process.inputStream, StandardCharsets.UTF_8)).use { reader ->
-            reader.lines().filter { it.isNotEmpty() }.toList()
-        }
-        ProcessGroovyMethods.closeStreams(process)
-        process.waitForOrThrow("Failed to run command")
-        return out
-    }
-
     fun onPath(name: String): Boolean {
         val path = System.getenv("PATH") ?: return false
         return path.splitToSequence(File.pathSeparator).any { File(it, name).exists() }