machine: ssh: Parameterize '%current-system' early on.

Fixes <https://issues.guix.gnu.org/58084>.
Reported by Maxim Cournoyer <maxim.cournoyer@gmail.com>.

Previously, "sanity checks" and other operations would happen in a
context where '%current-system' has its default value.  Thus, running
'guix deploy' on x86_64-linux machine for an aarch64-linux one would
lead things like '%base-initrd-modules' to see "x86_64-linux" as the
'%current-system' value, in turn making the wrong choices.

* gnu/machine/ssh.scm (check-deployment-sanity)[assertions]: Wrap in
'parameterize'.
(deploy-managed-host): Likewise for the 'mlet' body.

gnu/machine/ssh.scm

@@ -339,9 +339,13 @@ by MACHINE."
   "Raise a '&message' error condition if it is clear that deploying MACHINE's
 'system' declaration would fail."
   (define assertions
+    (parameterize ((%current-system
+                    (machine-ssh-configuration-system
+                     (machine-configuration machine)))
+                   (%current-target-system #f))
       (append (machine-check-file-system-availability machine)
               (machine-check-initrd-modules machine)
-            (list (machine-check-forward-update machine))))
+              (list (machine-check-forward-update machine)))))
 
   (define aggregate-exp
     ;; Gather all the expressions so that a single round-trip is enough to
@@ -453,6 +457,10 @@ the 'should-roll-back' field set to SHOULD-ROLL-BACK?"
 (define (deploy-managed-host machine)
   "Internal implementation of 'deploy-machine' for MACHINE instances with an
 environment type of 'managed-host."
+  (define config (machine-configuration machine))
+  (define host   (machine-ssh-configuration-host-name config))
+  (define system (machine-ssh-configuration-system config))
+
   (maybe-raise-unsupported-configuration-error machine)
   (when (machine-ssh-configuration-authorize?
          (machine-configuration machine))
@@ -466,11 +474,14 @@ have you run 'guix archive --generate-key?'")
                                        (get-string-all port))))
                                   (machine-ssh-session machine)
                                   (machine-become-command machine)))
+
   (mlet %store-monad ((_ (check-deployment-sanity machine))
                       (boot-parameters (machine-boot-parameters machine)))
+    ;; Make sure code that check %CURRENT-SYSTEM, such as
+    ;; %BASE-INITRD-MODULES, gets to see the right value.
+    (parameterize ((%current-system system)
+                   (%current-target-system #f))
       (let* ((os (machine-operating-system machine))
-           (host (machine-ssh-configuration-host-name
-                  (machine-configuration machine)))
              (eval (cut machine-remote-eval machine <>))
              (menu-entries (map boot-parameters->menu-entry boot-parameters))
              (bootloader-configuration (operating-system-bootloader os))
@@ -482,7 +493,8 @@ have you run 'guix archive --generate-key?'")
             (lambda (store)
               (guard (condition ((inferior-exception? condition)
                                  (values (begin handler ...) store)))
-              (values (run-with-store store (eval exp))
+                (values (run-with-store store (eval exp)
+                                        #:system system)
                         store)))))
 
         (mbegin %store-monad
@@ -509,7 +521,7 @@ an error occurred while upgrading services on '~a':~%~{~s ~}~%")
 failed to install bootloader on '~a':~%~{~s ~}~%")
                                             host
                                             (inferior-exception-arguments c))))
-                                bootloader-configuration bootcfg)))))))
+                                  bootloader-configuration bootcfg))))))))
 
 
 ;;;