Revert "appveyor: use rustc nightly instead of master"

This reverts commit 3f01619.

Author: matthiaskrgr

appveyor.yml

@@ -20,12 +20,11 @@ install:
     - set PATH=%PATH%;C:\Users\appveyor\.cargo\bin
     - git ls-remote https://github.com/rust-lang/rust.git master | awk '{print $1}' >rustc-hash.txt
     - set /p RUSTC_HASH=<rustc-hash.txt
-# uncomment once https://github.com/rust-lang/rust/issues/55376 is fixed
-#    - del rust-toolchain
-#    - cargo install rustup-toolchain-install-master || echo "rustup-toolchain-install-master already installed"
-#    - rustup-toolchain-install-master %RUSTC_HASH% -f -n master
-#    - rustup default master
-#    - set PATH=%PATH%;C:\Users\appveyor\.rustup\toolchains\master\bin
+    - del rust-toolchain
+    - cargo install rustup-toolchain-install-master || echo "rustup-toolchain-install-master already installed"
+    - rustup-toolchain-install-master %RUSTC_HASH% -f -n master
+    - rustup default master
+    - set PATH=%PATH%;C:\Users\appveyor\.rustup\toolchains\master\bin
     - rustc -V
     - cargo -V