Disable rustup self-updating

`rustup` has a mechanism for self-updating, which triggers whenever you
request it to install a new toolchain. This is annoying to have happen in CI
though, since it increases build time for no benefit.

So let's disable this mechanism.

images/macos/scripts/build/install-rust.sh

@@ -15,6 +15,15 @@ rustup-init -y --no-modify-path --default-toolchain=stable --profile=minimal
 echo "Initialize environment variables..."
 CARGO_HOME=$HOME/.cargo
 
+echo "Disable rustup self-updating"
+# This is undesirable because:
+# - We will keep rustup updated (just like any other dependency).
+# - rustup does not need to be the latest and greatest.
+# - Self-updating in CI will introduce longer build times for no benefit.
+# - ... which becomes important because self-updating happens in various
+#   places, including `rustup toolchain install`.
+rustup set auto-self-update disable
+
 echo "Install common tools..."
 rustup component add rustfmt clippy
 

images/ubuntu/scripts/build/install-rust.sh

@@ -16,6 +16,16 @@ curl -fsSL https://sh.rustup.rs | sh -s -- -y --default-toolchain=stable --profi
 # Initialize environment variables
 source $CARGO_HOME/env
 
+# Disable rustup self-updating
+#
+# This is undesirable because:
+# - We will keep rustup updated (just like any other dependency).
+# - Rustup does not need to be the latest and greatest.
+# - Self-updating in CI will introduce longer build times for no benefit.
+# - ... which becomes important because self-updating happens in various
+#   places, including `rustup toolchain install`.
+rustup set auto-self-update disable
+
 # Install common tools
 rustup component add rustfmt clippy
 

images/windows/scripts/build/Install-Rust.ps1

@@ -28,6 +28,16 @@ Add-DefaultPathItem "%USERPROFILE%\.cargo\bin"
 # Add Rust binaries to the path
 $env:Path += ";$env:CARGO_HOME\bin"
 
+# Disable rustup self-updating
+#
+# This is undesirable because:
+# - We will keep rustup updated (just like any other dependency).
+# - rustup does not need to be the latest and greatest.
+# - Self-updating in CI will introduce longer build times for no benefit.
+# - ... which becomes important because self-updating happens in various
+#   places, including `rustup toolchain install`.
+rustup set auto-self-update disable
+
 # Add i686 target for building 32-bit binaries
 rustup target add i686-pc-windows-msvc