fix(fuzz): Fix env var name in fuzzing workflow

[aakoshh]

Description

Problem*

Resolves a typo in the env var name.

Summary*

Fixes the env var name for time budget in the nightly workflow by removing the _FORCE from it.

Also changed NOIR_ARBTEST_SEED to NOIR_AST_FUZZER_SEED.

Additional Context

Originally I picked NOIR_ARBTEST_SEED because arbtest already looks for ARBTEST_SEED, and I thought by adding a NOIR_ prefix it's generally easy to see why an env var exists.

It similarly looks for ARBTEST_BUDGET_MS; for a moment I thought for consistency I should change from NOIR_AST_FUZZER_BUDGET_SECS to NOIR_ARBTEST_BUDGET_MS, but then I thought in the off-chance that we would like to configure different budgets for different uses of arbtest, differentiating by e.g. NOIR_AST_FUZZER_BUDGET_SECS vs NOIR_SOMETHING_ELSE_BUDGET_SECS is better than a single value.

Since I did fall foul sometimes to mixing up NOIR_ARBTEST_ and NOIR_AST_FUZZER_ prefixes, I thought it's better to use a single variant. ARBTEST_SEED still works, though, if one prefers brevity.

Documentation*

Check one:

• No documentation needed.
• Documentation included in this PR.
• [For Experimental Features] Documentation to be submitted in a separate PR.

PR Checklist*

• I have tested the changes locally.
• I have formatted the changes with Prettier and/or cargo fmt on default settings.