ci: Fix cases where unset variables cause errors

(backport <rust-lang#4108>)
(cherry picked from commit f3bb382)

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-$TARGET"
@@ -22,18 +19,18 @@ else
     rustup default "$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 [ "$OS" = "windows" ]; then
-    if [ "$ARCH_BITS" = "i686" ]; then
+if [ "$os" = "windows" ]; then
+    if [ "${ARCH_BITS:-}" = "i686" ]; then
         echo "Install MinGW32"
         choco install mingw --x86 --force
     fi
@@ -44,7 +41,7 @@ if [ "$OS" = "windows" ]; then
     /usr/bin/find "C:\ProgramData\Chocolatey" -name "dllcrt2*"
     /usr/bin/find "C:\ProgramData\Chocolatey" -name "libmsvcrt*"
 
-    if [ -n "$ARCH_BITS" ]; then
+    if [ -n "${ARCH_BITS:-}" ]; then
         echo "Fix MinGW"
         for i in crt2.o dllcrt2.o libmingwex.a libmsvcrt.a ; do
             cp -f "/C/ProgramData/Chocolatey/lib/mingw/tools/install/mingw$ARCH_BITS/$ARCH-w64-mingw32/lib/$i" "$(rustc --print sysroot)/lib/rustlib/$TARGET/lib"

ci/run.sh

@@ -15,7 +15,7 @@ target="$1"
 #
 # It's assume that all images, when run with two disks, will run the `run.sh`
 # script from the second which we place inside.
-if [ "$QEMU" != "" ]; then
+if [ -n "${QEMU:-}" ]; then
     tmpdir=/tmp/qemu-img-creation
     mkdir -p "${tmpdir}"