stack: Change unexpected environment variable error

Make the error more specific by stating that it's caused by a specific
environment variable and not an environment as a whole.
Also don't escape the variable to make it more readable.

Signed-off-by: Paweł Gronowski <pawel.gronowski@docker.com>
(cherry picked from commit 012b779)
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

cli/command/stack/loader/loader.go

@@ -120,7 +120,7 @@ func buildEnvironment(env []string) (map[string]string, error) {
 
 		k, v, ok := strings.Cut(s, "=")
 		if !ok || k == "" {
-			return result, errors.Errorf("unexpected environment %q", s)
+			return result, errors.Errorf("unexpected environment variable '%s'", s)
 		}
 		// value may be set, but empty if "s" is like "K=", not "K".
 		result[k] = v