Make it easier to provide Develocity access key

[bigdaz]

In order to publish build scans to a Develocity instance, an access key must be made available for each Gradle build execution. Presently, this means:

1. Generating an access key and adding it as a GitHub Actions secret
2. Adding an env block to each workflow that sets the access key as an environment variable.

The gradle-build-action is uninvolved in this process: the env var is read directly by the Gradle Build Tool.

Instead, we could make the gradle-build-action aware of the DV access key. This would change the requirements to:

1. Generating an access key and adding it as a GitHub Actions secret
2. Passing the access key as a parameter to the gradle-build-action

The action would then either:

• Configure the necessary environment variable for the access key
• Add the access key to ~/.gradle/enterprise/keys.properties where it will be read by the DV plugin

The second option may be preferable, since environment variables are not automatically available to composite actions or reusable workflows.

[bigdaz]

In some cases, passing the secret to the gradle-build-action is actually less convenient than configuring an env var: this is because it's possible to configure an env-var for an entire workflow, whereas the action parameters need to be specified per-job. Unfortunately, there's no way to make gradle-build-action to respond directly to the presence of the secret: action executions are sandboxed and can only see values that are explicitly provided.

In fact, I'm not sure there's any case where this is actually more convenient: passing the secret to the gradle-build-action which then configures the env var (or writes a file) is not more convenient than configuring the env directly. The env can be specified at the Workflow, Job or Step level.