[CI] Fix RUSTFLAGS

[Jefffrey]

Which issue does this PR close?

Closes #.

Rationale for this change

4289737 sped win ci up, running at 33min:

• https://github.com/apache/arrow-datafusion/actions/runs/7433532490

Following commit dd4263f had runtime of 39min:

• https://github.com/apache/arrow-datafusion/actions/runs/7433765725

Subsequent runs fall around 39-42min range

Until this commit d9a1d42 which causes runtime of 47min:

• https://github.com/apache/arrow-datafusion/actions/runs/7494637547

Subsequent runs (up to latest main) fall around 44-48min range

What changes are included in this PR?

Fix accidental regression by d9a1d42 which named the env var incorrectly.

Are these changes tested?

Are there any user-facing changes?

[Jefffrey]

Two commit action runs with fix:

• https://github.com/apache/arrow-datafusion/actions/runs/7600594577 - 41min
• https://github.com/apache/arrow-datafusion/actions/runs/7609479932 - 42min

Fixes d9a1d42

For dd4263f I'm not sure if the slowdown was due to setting RUST_MIN_STACK or due to the code changes. If it was due to the RUST_MIN_STACK then hopefully should be resolved by #8837

[comphead]

thanks @Jefffrey for fixing that nasty typo

[comphead]

Basically CI times fluctuates, I saw Macos builds being 19 mins, and on the same codebase the next run 28mins.

[alamb]

Thanks @Jefffrey and @comphead