Pin nightly rust version to fix failing MIRI job (#8183)

# Which issue does this PR close?

- Related to #8181

# Rationale for this change

I am trying to get CI clean on main in preparation for a release, but
sadly the MIRI job started failing with an internal (rust) compiler
error

I believe this is due to the fact we are using bleeding edge rust
(nightly)

# What changes are included in this PR?

Temporarily pin the MIRI job to use nightly from last night rather than
now

# Are these changes tested?

Yes by CI

# Are there any user-facing changes?

No

Author: alamb

.github/workflows/miri.yaml

@@ -52,8 +52,12 @@ jobs:
           submodules: true
       - name: Setup Rust toolchain
         run: |
-          rustup toolchain install nightly --component miri
-          rustup override set nightly
+          # Temp pin to nightly-2025-08-18 until https://github.com/rust-lang/rust/issues/145652 is resolved
+          # See https://github.com/apache/arrow-rs/issues/8181 for more details
+          rustup toolchain install nightly-2025-08-18 --component miri
+          rustup override set nightly-2025-08-18
+          # rustup toolchain install nightly --component miri
+          # rustup override set nightly
           cargo miri setup
       - name: Run Miri Checks
         env: