[sled-agent] Make new mountpoints immutable (#7888)

Partial fix of #7874 and
#4203

This PR causes new points to be created as immutable directories,
preventing files from begin created
in the "parent filesystem", underneath the intended mountpoint.

Author: smklein

Cargo.lock

@@ -704,6 +704,7 @@ wicket = { path = "wicket" }
 wicket-common = { path = "wicket-common" }
 wicketd-api = { path = "wicketd-api" }
 wicketd-client = { path = "clients/wicketd-client" }
+xshell = "0.2.7"
 zeroize = { version = "1.8.1", features = ["zeroize_derive", "std"] }
 # NOTE: Avoid upgrading zip until https://github.com/zip-rs/zip2/issues/231 is resolved
 zip = { version = "=2.1.3", default-features = false, features = ["deflate","bzip2"] }