fix(token-from-stdout): Treat process spawn errors as unsupported

Like with `libsecret` not being present, process spawn errors can be
treated as an indication that that config entry isn't relevant for the
current machine and we should try other entries.

If there is nothing else to fallback to, we'll still report this error.

Author: epage

src/cargo/util/credential/adaptor.rs

@@ -7,7 +7,7 @@ use std::{
 
 use anyhow::Context;
 use cargo_credential::{
-    Action, CacheControl, Credential, CredentialResponse, RegistryInfo, Secret,
+    Action, CacheControl, Credential, CredentialResponse, Error, ErrorKind, RegistryInfo, Secret,
 };
 
 pub struct BasicProcessCredential {}
@@ -33,7 +33,10 @@ impl Credential for BasicProcessCredential {
                     cmd.env("CARGO_REGISTRY_NAME_OPT", name);
                 }
                 cmd.stdout(Stdio::piped());
-                let mut child = cmd.spawn().context("failed to spawn credential process")?;
+                let mut child = cmd
+                    .spawn()
+                    .context("failed to spawn credential process")
+                    .map_err(|err| Error::from(err).with_kind(ErrorKind::UrlNotSupported))?;
                 let mut buffer = String::new();
                 child
                     .stdout