Revert "temporarily ignore cached rustup-toolchain-install-master"

This reverts commit 4872c5c.

Author: RalfJung

.appveyor.yml

@@ -31,7 +31,7 @@ install:
     - rustup component remove rust-docs & exit 0
     - rustup update
     # Install "master" toolchain
-    - cargo install rustup-toolchain-install-master -f
+    - cargo install rustup-toolchain-install-master & exit 0
     # We need to install cargo here as well or else the DLL search path inside `cargo run`
     # will be for the wrong toolchain. (On Unix, `./miri` takes care of this, but not here.)
     - rustup-toolchain-install-master -f -n master %RUSTC_HASH% -c rust-src -c rustc-dev -c cargo

.travis.yml

@@ -38,7 +38,7 @@ before_script:
 - rustup component remove rust-docs || echo "rust-docs already gone"
 - rustup update
 # Install "master" toolchain
-- cargo install rustup-toolchain-install-master -f
+- cargo install rustup-toolchain-install-master || echo "rustup-toolchain-install-master already installed"
 - travis_retry rustup-toolchain-install-master -f -n master $RUSTC_HASH -c rust-src -c rustc-dev
 - rustup default master
 - rustc --version