Add an env variable to allow skipping the call to the github api

Github's API likes not responding to some requests. Let's see if we can
just skip it without issue.

src/toolchain/rust.rs

@@ -111,6 +111,10 @@ impl XtensaRust {
 
     /// Parses the version of the Xtensa toolchain.
     pub fn parse_version(arg: &str) -> Result<String, Error> {
+        if std::env::var_os("ESPUP_SKIP_VERSION_PARSE").is_some() {
+            return Ok(arg.to_string());
+        }
+
         debug!("{} Parsing Xtensa Rust version: {}", emoji::DEBUG, arg);
         let re_extended = Regex::new(RE_EXTENDED_SEMANTIC_VERSION).unwrap();
         let re_semver = Regex::new(RE_SEMANTIC_VERSION).unwrap();