polyval: pin nightly version used for ARMv8 testing

The build is currently failing:

https://github.com/RustCrypto/universal-hashes/runs/5629586918?check_suite_focus=true

It's for two reasons:

1. `aarch64_target_feature` was recently stabilized in
   rust-lang/rust#90621 but we still include it
2. There's a bug releating to `neon`/`fp` activation. See
   rust-lang/rust#91608 and rust-lang/rust#95044

Until one of the fixes in the second issue is merged, we can't really
make progress. So this commit pins to `nightly-2022-03-01` so we can
continue to have a clean build.

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

.github/workflows/ghash.yml

@@ -3,6 +3,7 @@ name: ghash
 on:
   pull_request:
     paths:
+      - ".github/workflows/ghash.yml"
       - "ghash/**"
       - "Cargo.*"
   push:

.github/workflows/poly1305.yml

@@ -3,6 +3,7 @@ name: poly1305
 on:
   pull_request:
     paths:
+      - ".github/workflows/poly1305.yml"
       - "poly1305/**"
       - "Cargo.*"
   push:

.github/workflows/polyval.yml

@@ -3,6 +3,7 @@ name: polyval
 on:
   pull_request:
     paths:
+      - ".github/workflows/polyval.yml"
       - "polyval/**"
       - "Cargo.*"
   push:
@@ -187,7 +188,7 @@ jobs:
       matrix:
         include:
           - target: aarch64-unknown-linux-gnu
-            rust: nightly
+            rust: nightly-2022-03-01
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v1