Since adding these to the `org.gradle.java.installations.fromEnv` property
is problematic (#1024), this mechanism allows the default toolchains to
be discovered by Gradle via a different mechanism.
The default JDK installations are added to `~/.m2/toolchains.xml` such that
they are discoverable by Gradle toolchain support.
The `setup-java` action also writes to this file, so we merge with any existing
content: this allows both pre-installed and "setup" JDKs to be automatically
detected by Gradle.