Set caller to suppress ssh configuration messages

They are offered both when the ssh session works but the called
program returns error and when control-c ends a session. Not really’
useful information in either case.

Change-Id: I4472694b2b48bba40813c7c303c8f580904b7329
Reviewed-on: https://fuchsia-review.googlesource.com/c/fargo/+/458157
Reviewed-by: Dan Johnson <computerdruid@google.com>
diff --git a/src/device.rs b/src/device.rs
index 4ce7f4a..7c0b7ab 100644
--- a/src/device.rs
+++ b/src/device.rs
@@ -45,6 +45,7 @@
     }
 
     ssh_cmd.arg("shell").arg(command);
+    ssh_cmd.env("FX_CALLER", "fargo");
 
     if verbose {
         println!("ssh_cmd = {:#?}", ssh_cmd);