azure-pipelines: use rust version defined in rust-toolchain file

.azure/azure-pipelines.yml

@@ -1,6 +1,3 @@
-variables:
-  lkg-rust-nightly: "2019-08-22"
-
 trigger:
   - master
 
@@ -8,13 +5,10 @@ strategy:
   matrix:
     linux-nightly:
       image: ubuntu-16.04
-      toolchain: nightly-$(lkg-rust-nightly)
     macos-nightly:
       image: macos-10.13
-      toolchain: nightly-$(lkg-rust-nightly)
     windows-nightly:
       image: vs2017-win2016
-      toolchain: nightly-$(lkg-rust-nightly)
 
 pool:
   vmImage: $(image)
@@ -22,10 +16,8 @@ pool:
 steps:
   - bash: |
       set -e
-      curl https://sh.rustup.rs -sSf | sh -s -- -y --no-modify-path --default-toolchain none
+      curl https://sh.rustup.rs -sSf | sh -s -- -y --no-modify-path --default-toolchain `cat rust-toolchain`
       export PATH=$HOME/.cargo/bin:$PATH
-      rustup install --no-self-update $(toolchain)
-      rustup default $(toolchain)
       rustc -Vv
       echo "##vso[task.prependpath]$HOME/.cargo/bin"
     displayName: Install Rust