Add MSRV validation GitHub Action for cargo-credential

Author: arlosi

.github/workflows/main.yml

@@ -246,3 +246,10 @@ jobs:
         cd target
         curl -sSLO https://raw.githubusercontent.com/rust-lang/rust/master/src/tools/linkchecker/linkcheck.sh
         sh linkcheck.sh --all --path ../src/doc cargo
+
+  credential_msrv:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v3
+    - run: rustup update 1.70 && rustup default 1.70
+    - run: cargo test -p cargo-credential

credential/cargo-credential/Cargo.toml

@@ -3,6 +3,7 @@ name = "cargo-credential"
 version = "0.3.1"
 edition.workspace = true
 license.workspace = true
+rust-version = "1.70.0"
 repository = "https://github.com/rust-lang/cargo"
 description = "A library to assist writing Cargo credential helpers."