Set GOMAXPROCS when parallelism is configured

**What would you like to be added**:
In order to make behavior more predictable, if a user configures parallelism, it might be a good idea to set [GOMAXPROCS](https://pkg.go.dev/runtime#GOMAXPROCS) to limit the number of system threads.

**Why is this needed**:
More predictable performance characteristics.

**Additional context**:
This stemmed from a [discussion on discourse](https://anchorecommunity.discourse.group/t/limiting-the-number-of-threads-used/442).

No source file or test is named. Start by tracing where configured parallelism is parsed and applied, then read the linked GOMAXPROCS documentation and discourse discussion. Done means the configured behavior is implemented with clear semantics and verified by relevant tests.

Written by the indexing model from the issue text.