test_infra: Enable cross-build for "musl" and "aarch64" targets

With enabling the `vendered-openssl` feature, we can now cross-build the
`test_infra` crate for  "musl" and "aarch64" targets. In this way, we
can remove the `test_infra` crate from the "exclude" list, so that this
crate can be checked and processed by "cargo clippy" and "cargo fmt".

More details can be found: https://docs.rs/openssl/latest/openssl/#vendored

As 'musl-gcc' is required, this commit also installs the `musl-tools`
package for our "build" github action on the musl target [1].

[1] https://github.com/actions-rs/toolchain/issues/102

Signed-off-by: Bo Chen <chen.bo@intel.com>

test_infra/Cargo.toml

@@ -9,6 +9,6 @@ dirs = "3.0.1"
 epoll = "4.3.1"
 lazy_static= "1.4.0"
 libc = "0.2.91"
-ssh2 = "0.9.1"
+ssh2 = { version = "0.9.1", features = ["vendored-openssl"]}
 vmm-sys-util = "0.9.0"
 wait-timeout = "0.2.0"