Remove the acme_common build script's warning

acme_common/build.rs

@@ -7,10 +7,10 @@ macro_rules! set_rustc_env_var {
 }
 
 fn main() {
-    if let Ok(_) = env::var("DEP_OPENSSL_VERSION_NUMBER") {
+    if env::var("DEP_OPENSSL_VERSION_NUMBER").is_ok() {
         set_rustc_env_var!("ACMED_TLS_LIB_NAME", "OpenSSL");
     }
-    if let Ok(_) = env::var("DEP_OPENSSL_LIBRESSL_VERSION_NUMBER") {
+    if env::var("DEP_OPENSSL_LIBRESSL_VERSION_NUMBER").is_ok() {
         set_rustc_env_var!("ACMED_TLS_LIB_NAME", "LibreSSL");
     }
 }