minimal-versions: replace workspace Cargo.toml instead of deleting (#51)

Generates a stub Cargo.toml at the workspace level which configures a
workspace with one member whose path is the current working directory.

I think this might resolve the problem I was having with using
workspace-level configs in conjunction with `minimal-versions` on
RustCrypto/utils#1411

Not sure how to test it

Author: tarcieri

.github/workflows/minimal-versions.yml

@@ -46,7 +46,8 @@ jobs:
       - uses: dtolnay/rust-toolchain@master
         with:
           toolchain: ${{ inputs.nightly }}
-      - run: rm ../Cargo.toml
+        # Use a stub Cargo.toml at the workspace-level which still allows workspace-level configs to function
+      - run: printf "[workspace]\nmembers=[\"%q\"]" $(pwd) >> ../Cargo.toml
       - run: cargo update -Z minimal-versions
       - run: ${{ inputs.nightly-cmd }}
         # Perform tests

.yamllint.yaml

@@ -0,0 +1,2 @@
+rules:
+  line-length: disable