ci: set -u in cirrus.sh to treat unset variables as an error

.cirrus.yml

@@ -1,6 +1,7 @@
 env:
   ### compiler options
   HOST:
+  WRAPPER_CMD:
   # Specific warnings can be disabled with -Wno-error=foo.
   # -pedantic-errors is not equivalent to -Werror=pedantic and thus not implied by -Werror according to the GCC manual.
   WERROR_CFLAGS: -Werror -pedantic-errors

ci/cirrus.sh

@@ -1,7 +1,6 @@
 #!/bin/sh
 
-set -e
-set -x
+set -eux
 
 export LC_ALL=C
 
@@ -16,10 +15,21 @@ print_environment() {
             EXPERIMENTAL ECDH RECOVERY SCHNORRSIG \
             SECP256K1_TEST_ITERS BENCH SECP256K1_BENCH_ITERS CTIMETEST\
             EXAMPLES \
-            WRAPPER_CMD CC AR NM HOST
+            HOST WRAPPER_CMD
     do
         eval 'printf "%s %s " "$i=\"${'"$i"'}\""'
     done
+    # In contrast to the previous variables, CC, AR and NM may be unset. Print
+    # them if they are set (even if set to the empty string).
+    if [ -n "${CC+x}" ]; then
+        echo -n "CC=\"${CC}\" "
+    fi
+    if [ -n "${AR+x}" ]; then
+        echo -n "AR=\"${AR}\" "
+    fi
+    if [ -n "${NM+x}" ]; then
+        echo -n "NM=\"${NM}\" "
+    fi
     echo "$0"
     set -x
 }
@@ -36,7 +46,7 @@ esac
 
 env >> test_env.log
 
-if [ -n "$CC" ]; then
+if [ -n "${CC+x}" ]; then
     # The MSVC compiler "cl" doesn't understand "-v"
     $CC -v || true
 fi