Dependency-submission fails to find workflow files when DEPENDENCY_GRAPH_REPORT_DIR is specified

[bigdaz]

If set, the DEPENDENCY_GRAPH_REPORT_DIR env var will control where the GitHub Dependency Graph Gradle Plugin generates it's reports. The dependency-submission action sets to the <root>/dependency-graph-reports by default.

However, the action implementation currently assumes that the default value is used, and will not locate the generated files if the report dir is set to another location.

We should:

• Fix the action so that setting DEPENDENCY_GRAPH_REPORT_DIR works for generating and submitting dependency graph files.
• Possibly add a reports-dir parameter to the action so it's more obvious how to override the report output directory.