Switch to specific rust nightly version

Until macOS build is fixed, see
rust-lang/rust#76698

Explain rust version requirements in README.

.github/workflows/build.yml

@@ -34,8 +34,10 @@ jobs:
     - name: Set Rustup profile to minimal
       run: rustup set profile minimal
 
-    - name: "Switch to Rust nightly"
-      run: rustup default nightly
+    - name: "Switch to Rust nightly-2020-09-11"
+      run: |
+        rustup install nightly-2020-09-11
+        rustup default nightly-2020-09-11
 
     - name: "Print Rust Version"
       run: |

README.md

@@ -30,7 +30,9 @@ Vesper has been influenced by the kernels in L4 family, notably seL4. Fawn and N
 
 ## Build instructions
 
-Use rustc nightly 2020-07-15 with cargo nightly of the same or later date.
+Use at least rustc nightly 2020-07-15 with cargo nightly of the same or later date. It adds support for `cargo build --build-std` feature.
+
+Currently, we are bound to version not later than `nightly-2020-09-11` until [this bug](https://github.com/rust-lang/rust/issues/76698) is fixed for macOS.
 
 Install tools: `cargo install just cargo-make`.
 Install qemu (at least version 4.1.1): `brew install qemu`.

rust-toolchain

@@ -1 +1 @@
-nightly
+nightly-2020-09-11