Create toolchains.xml dynamically based on envs (#150)

Different runners have different JDKs installed, so using a hard-coded
list for
`toolchains.xml` doesn't work. With this change, the file is generated
based on the available `JAVA_HOME_*` environment variables.

Fixes #89 

Thanks @hfhbd for the contribution!

Co-authored-by: hfhbd <22521688+hfhbd@users.noreply.github.com>

sources/src/caching/gradle-user-home-utils.ts

@@ -0,0 +1,44 @@
+import path from 'path'
+import fs from 'fs'
+
+export function readResourceFileAsString(...paths: string[]): string {
+    // Resolving relative to __dirname will allow node to find the resource at runtime
+    const absolutePath = path.resolve(__dirname, '..', '..', '..', 'sources', 'src', 'resources', ...paths)
+    return fs.readFileSync(absolutePath, 'utf8')
+}
+
+/**
+ * Iterate over all `JAVA_HOME_{version}_{arch}` envs and construct the toolchain.xml.
+ *
+ * @VisibleForTesting
+ */
+export function getPredefinedToolchains(): string | null {
+    const javaHomeEnvs: string[] = []
+    for (const javaHomeEnvsKey in process.env) {
+        if (javaHomeEnvsKey.startsWith('JAVA_HOME_')) {
+            javaHomeEnvs.push(javaHomeEnvsKey)
+        }
+    }
+    if (javaHomeEnvs.length === 0) {
+        return null
+    }
+    // language=XML
+    let toolchainsXml = `<?xml version="1.0" encoding="UTF-8"?>
+<toolchains>
+<!-- JDK Toolchains installed by default on GitHub-hosted runners -->
+`
+    for (const javaHomeEnv of javaHomeEnvs) {
+        const version = javaHomeEnv.match(/JAVA_HOME_(\d+)_/)?.[1]!
+        toolchainsXml += `  <toolchain>
+    <type>jdk</type>
+    <provides>
+      <version>${version}</version>
+    </provides>
+    <configuration>
+      <jdkHome>\${env.${javaHomeEnv}}</jdkHome>
+    </configuration>
+  </toolchain>\n`
+    }
+    toolchainsXml += `</toolchains>\n`
+    return toolchainsXml
+}