travis: Move cloudabi to xbuild section

Cloud ABI [is now Tier 3](rust-lang/rust#75568),
so its libstd is no longer distributed with Rust.

Signed-off-by: Joe Richey <joerichey@google.com>

.travis.yml

@@ -129,7 +129,6 @@ matrix:
       rust: nightly-2020-09-08
       install:
         - rustup target add x86_64-sun-solaris
-        - rustup target add x86_64-unknown-cloudabi
         - rustup target add x86_64-unknown-freebsd
         - rustup target add x86_64-fuchsia
         - rustup target add x86_64-unknown-netbsd
@@ -140,25 +139,25 @@ matrix:
         - cargo install cargo-xbuild || true
       script:
         - cargo build --target=x86_64-sun-solaris
-        - cargo build --target=x86_64-unknown-cloudabi
         - cargo build --target=x86_64-unknown-freebsd
         - cargo build --target=x86_64-fuchsia
         - cargo build --target=x86_64-unknown-netbsd
         - cargo build --target=x86_64-unknown-redox
         - cargo build --target=x86_64-fortanix-unknown-sgx
+        - cargo xbuild --target=x86_64-unknown-cloudabi
         - cargo xbuild --target=x86_64-unknown-uefi
         - cargo xbuild --target=x86_64-unknown-hermit
         - cargo xbuild --target=x86_64-unknown-l4re-uclibc
         - cargo xbuild --target=x86_64-wrs-vxworks
         # also test minimum dependency versions are usable
         - cargo generate-lockfile -Z minimal-versions
         - cargo build --target=x86_64-sun-solaris
-        - cargo build --target=x86_64-unknown-cloudabi
         - cargo build --target=x86_64-unknown-freebsd
         - cargo build --target=x86_64-fuchsia
         - cargo build --target=x86_64-unknown-netbsd
         - cargo build --target=x86_64-unknown-redox
         - cargo build --target=x86_64-fortanix-unknown-sgx
+        - cargo xbuild --target=x86_64-unknown-cloudabi
         - cargo xbuild --target=x86_64-unknown-uefi
         - cargo xbuild --target=x86_64-unknown-hermit
         - cargo xbuild --target=x86_64-unknown-l4re-uclibc