contrib: install Rust 1.46.0

contrib: install Rust 1.46.0

Let's use the latest/greatest version of Rust in the automation
environment.

Differential Revision: https://phab.mercurial-scm.org/D9146

--- a/contrib/automation/hgautomation/linux.py	Sat Oct 03 19:52:52 2020 -0700
+++ b/contrib/automation/hgautomation/linux.py	Sat Oct 03 14:44:11 2020 -0700
@@ -72,7 +72,7 @@
 
 chmod +x rustup-init
 sudo -H -u hg -g hg ./rustup-init -y
-sudo -H -u hg -g hg /home/hg/.cargo/bin/rustup install 1.31.1 1.42.0
+sudo -H -u hg -g hg /home/hg/.cargo/bin/rustup install 1.31.1 1.46.0
 sudo -H -u hg -g hg /home/hg/.cargo/bin/rustup component add clippy
 
 sudo -H -u hg -g hg /home/hg/.cargo/bin/cargo install --version 0.7.0 pyoxidizer

--- a/contrib/install-windows-dependencies.ps1	Sat Oct 03 19:52:52 2020 -0700
+++ b/contrib/install-windows-dependencies.ps1	Sat Oct 03 14:44:11 2020 -0700
@@ -126,7 +126,7 @@
 
     Invoke-Process "${prefix}\assets\rustup-init.exe" "-y --default-host x86_64-pc-windows-msvc"
     Invoke-Process "${prefix}\cargo\bin\rustup.exe" "target add i686-pc-windows-msvc"
-    Invoke-Process "${prefix}\cargo\bin\rustup.exe" "install 1.42.0"
+    Invoke-Process "${prefix}\cargo\bin\rustup.exe" "install 1.46.0"
     Invoke-Process "${prefix}\cargo\bin\rustup.exe" "component add clippy"
 
     # Install PyOxidizer for packaging.