ci: Fix cases where unset variables cause errors

Author: tgross35

ci/install-rust.sh

@@ -4,14 +4,11 @@
 set -eux
 
 echo "Setup toolchain"
-toolchain=
-if [ -n "$TOOLCHAIN" ]; then
-  toolchain=$TOOLCHAIN
-else
-  toolchain=nightly
-fi
 
-if [ "$OS" = "windows" ]; then
+toolchain="${TOOLCHAIN:-nightly}"
+os="${OS:-}"
+
+if [ "$os" = "windows" ]; then
     : "${TARGET?The TARGET environment variable must be set.}"
     rustup set profile minimal
     rustup update --force "$toolchain-$TARGET"
@@ -22,17 +19,17 @@ else
     rustup default "$toolchain"
 fi
 
-if [ -n "$TARGET" ]; then
+if [ -n "${TARGET:-}" ]; then
     echo "Install target"
     rustup target add "$TARGET"
 fi
 
-if [ -n "$INSTALL_RUST_SRC" ]; then
+if [ -n "${INSTALL_RUST_SRC:-}" ]; then
     echo "Install rust-src"
     rustup component add rust-src
 fi
 
-if [ "$OS" = "windows" ]; then
+if [ "$os" = "windows" ]; then
     if [ "$ARCH_BITS" = "i686" ]; then
         echo "Install MinGW32"
         choco install mingw --x86 --force