Update nightly compiler on CI

The old one cannot build the `semver` dependency.

Author: ilya-biryukov

.github/workflows/rust.yml

@@ -158,7 +158,7 @@ jobs:
     strategy:
       matrix:
         arch: [amd64]
-        rust: [nightly-2021-03-24]
+        rust: [nightly-2021-07-04]
     container:
       image: ${{ matrix.arch }}/rust
       env:
@@ -275,7 +275,7 @@ jobs:
     strategy:
       matrix:
         arch: [amd64]
-        rust: [nightly-2021-03-24]
+        rust: [nightly-2021-07-04]
     steps:
       - uses: actions/checkout@v2
         with:
@@ -400,7 +400,7 @@ jobs:
     strategy:
       matrix:
         arch: [amd64]
-        rust: [nightly-2021-03-24]
+        rust: [nightly-2021-07-04]
     container:
       image: ${{ matrix.arch }}/rust
       env: