Pin nightly version

The build is currently failing:

<https://github.com./image-rs/jpeg-decoder/runs/5618199478>

1. `aarch64_target_feature` was stabilized in rust-lang/rust#90621
2. `neon`/`fp` must be activated together, which is not yet the case for
   some intrinsics in `std`. See rust-lang/rust#91608 and
   rust-lang/rust#95044.

Once either of the above solutions lands we can remove
`aarch64_target_feature` and unpin nightly again.

Author: HeroicKatora

.github/workflows/rust.yml

@@ -43,7 +43,7 @@ jobs:
 
     strategy:
       matrix:
-        rust: [nightly]
+        rust: [nightly-2022-03-08]
         features: ["", "rayon", "nightly_aarch64_neon"]
 
     steps: