Switch CI back to nightly channel (#1503)

Looks like `proptest` has been updated so we can move back to the
`nightly` channel rather than pinning.

.github/workflows/main.yml

@@ -52,7 +52,7 @@ jobs:
         submodules: true
     - uses: ./.github/actions/install-rust
       with:
-        toolchain: nightly-2020-04-07
+        toolchain: nightly
     - run: cargo doc --no-deps --all --exclude wasmtime-cli --exclude test-programs --exclude cranelift-codegen-meta
     - run: cargo doc --package cranelift-codegen-meta --document-private-items
     - uses: actions/upload-artifact@v1
@@ -72,7 +72,7 @@ jobs:
         submodules: true
     - uses: ./.github/actions/install-rust
       with:
-        toolchain: nightly-2020-04-07
+        toolchain: nightly
 
     # Check some feature combinations of the `wasmtime` crate
     - run: cargo check --manifest-path crates/api/Cargo.toml --no-default-features
@@ -121,7 +121,7 @@ jobs:
         ref: refs/heads/master
     - uses: ./.github/actions/install-rust
       with:
-        toolchain: nightly-2020-04-07
+        toolchain: nightly
     - run: cargo install cargo-fuzz --vers "^0.7"
     - run: cargo fetch
       working-directory: ./fuzz
@@ -182,7 +182,7 @@ jobs:
             rust: beta
           - build: nightly
             os: ubuntu-latest
-            rust: nightly-2020-04-07
+            rust: nightly
           - build: macos
             os: macos-latest
             rust: stable

Cargo.lock

@@ -1336,9 +1336,9 @@ dependencies = [
 
 [[package]]
 name = "proptest"
-version = "0.9.5"
+version = "0.9.6"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "bf6147d103a7c9d7598f4105cf049b15c99e2ecd93179bf024f0fd349be5ada4"
+checksum = "01c477819b845fe023d33583ebf10c9f62518c8d79a0960ba5c36d6ac8a55a5b"
 dependencies = [
  "bit-set",
  "bitflags",
@@ -1943,18 +1943,18 @@ dependencies = [
 
 [[package]]
 name = "thiserror"
-version = "1.0.14"
+version = "1.0.15"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "f0570dc61221295909abdb95c739f2e74325e14293b2026b0a7e195091ec54ae"
+checksum = "54b3d3d2ff68104100ab257bb6bb0cb26c901abe4bd4ba15961f3bf867924012"
 dependencies = [
  "thiserror-impl",
 ]
 
 [[package]]
 name = "thiserror-impl"
-version = "1.0.14"
+version = "1.0.15"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "227362df41d566be41a28f64401e07a043157c21c14b9785a0d8e256f940a8fd"
+checksum = "ca972988113b7715266f91250ddb98070d033c62a011fa0fcc57434a649310dd"
 dependencies = [
  "proc-macro2",
  "quote",