matrix: don't overwrite matrix eval context on attr only matrices

[ryancragun]

Matrix blocks are used in several places and depending on the context we we need slightly different behavior when decoding them.

When we're decoding a "full matrix", i.e. one that must have variants and may have include and exclude sub-blocks, we want to first decode the attributes and update the matrix eval context so that it's available when decoding the next attribute or the the later blocks.

When we're decoding include or exclude blocks, they themselves are also decoded as an attrubute only matrix, but we don't want to update the matrix eval context and instead want to inherit the parent matrix's eval context.

This change makes that so, fixing a bug whereby the matrix block only included the parents eval context on the first include or exclude block.

We also end up pinning the version of random in our acceptance tests because a newly released version broke the test output that we expect. Since we only care about getting a diagnostic warning it should be fine to pin to that version.

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