Set rustup default toolchain and export RUSTUP_HOME

After installing Rust, we need to explicitly set the default toolchain
with 'rustup default stable' so that cargo and other Rust tools can
find the toolchain to use.

Also added RUSTUP_HOME to the environment configuration so it's
properly set and persisted across shell sessions. This ensures rustup
can locate its installation and toolchain data.

Fixes the error: 'rustup could not choose a version of cargo to run,
because one wasn't specified explicitly, and no default is configured.'

Author: aclark4life

.evergreen/scripts/configure-env.sh

@@ -14,6 +14,7 @@ fi
 PROJECT_DIRECTORY="$(pwd)"
 DRIVERS_TOOLS="$(dirname $PROJECT_DIRECTORY)/drivers-tools"
 CARGO_HOME=${CARGO_HOME:-${DRIVERS_TOOLS}/.cargo}
+RUSTUP_HOME=${RUSTUP_HOME:-${CARGO_HOME}}
 UV_TOOL_DIR=$PROJECT_DIRECTORY/.local/uv/tools
 UV_CACHE_DIR=$PROJECT_DIRECTORY/.local/uv/cache
 DRIVERS_TOOLS_BINARIES="$DRIVERS_TOOLS/.bin"
@@ -34,6 +35,7 @@ if [ "Windows_NT" = "${OS:-}" ]; then # Magic variable in cygwin
     DRIVERS_TOOLS=$(cygpath -m $DRIVERS_TOOLS)
     PROJECT_DIRECTORY=$(cygpath -m $PROJECT_DIRECTORY)
     CARGO_HOME=$(cygpath -m $CARGO_HOME)
+    RUSTUP_HOME=$(cygpath -m $RUSTUP_HOME)
     UV_TOOL_DIR=$(cygpath -m "$UV_TOOL_DIR")
     UV_CACHE_DIR=$(cygpath -m "$UV_CACHE_DIR")
     DRIVERS_TOOLS_BINARIES=$(cygpath -m "$DRIVERS_TOOLS_BINARIES")
@@ -62,6 +64,7 @@ export DRIVERS_TOOLS_BINARIES="$DRIVERS_TOOLS_BINARIES"
 export PROJECT_DIRECTORY="$PROJECT_DIRECTORY"
 
 export CARGO_HOME="$CARGO_HOME"
+export RUSTUP_HOME="$RUSTUP_HOME"
 export UV_TOOL_DIR="$UV_TOOL_DIR"
 export UV_CACHE_DIR="$UV_CACHE_DIR"
 export UV_TOOL_BIN_DIR="$DRIVERS_TOOLS_BINARIES"

.evergreen/scripts/install-rust.sh

@@ -51,6 +51,10 @@ else
     cargo --version
 fi
 
+# Ensure default toolchain is set (needed for rustup to work properly)
+echo "Setting default toolchain to stable..."
+rustup default stable
+
 # Install maturin if not already installed
 if ! command -v maturin &> /dev/null; then
     echo "Installing maturin..."