Try to install only available toolchains via rustup

This prevents cross from erroring out and exiting when we have a custom
target which we know will be available in the docker image (but is not
one of the standard rustup targets). An example of such a target is
"x86_64-alpine-linux-musl" which is a custom target that the alpine
distro uses and is available when using rustc from its repositories.

src/main.rs

@@ -256,7 +256,10 @@ fn run() -> Result<ExitStatus> {
             }
             .unwrap_or_else(|| !target.is_builtin() || !available_targets.contains(&target));
 
-            if !uses_xargo && !available_targets.is_installed(&target) {
+            if !uses_xargo
+                && !available_targets.is_installed(&target)
+                && available_targets.contains(&target)
+            {
                 rustup::install(&target, &toolchain, verbose)?;
             } else if !rustup::component_is_installed("rust-src", &toolchain, verbose)? {
                 rustup::install_component("rust-src", &toolchain, verbose)?;