services: guix-publish: Add 'cache-bypass-threshold' field.

This is a followup to ecaa102a58ad3ab0b42e04a3d10d7c761c05ec98.

* gnu/services/base.scm (<guix-publish-configuration>)[cache-bypass-threshold]:
New field.
(guix-publish-shepherd-service): Honor it.

doc/guix.texi

@@ -15041,6 +15041,12 @@ When it is an integer, this is the number of worker threads used for
 caching; when @code{#f}, the number of processors is used.
 @xref{Invoking guix publish, @option{--workers}}, for more information.
 
+@item @code{cache-bypass-threshold} (default: 10 MiB)
+When @code{cache} is true, this is the maximum size in bytes of a store
+item for which @command{guix publish} may bypass its cache in case of a
+cache miss.  @xref{Invoking guix publish,
+@option{--cache-bypass-threshold}}, for more information.
+
 @item @code{ttl} (default: @code{#f})
 When it is an integer, this denotes the @dfn{time-to-live} in seconds
 of the published archives.  @xref{Invoking guix publish, @option{--ttl}},

gnu/services/base.scm

@@ -1740,6 +1740,8 @@ proxy of 'guix-daemon'...~%")
                (default "nar"))
   (cache       guix-publish-configuration-cache   ;#f | string
                (default #f))
+  (cache-bypass-threshold guix-publish-configuration-cache-bypass-threshold
+                          (default (* 10 (expt 2 20)))) ;integer
   (workers     guix-publish-configuration-workers ;#f | integer
                (default #f))
   (ttl         guix-publish-configuration-ttl     ;#f | integer
@@ -1774,7 +1776,7 @@ raise a deprecation warning if the 'compression-level' field was used."
                    lst))))
 
   (match-record config <guix-publish-configuration>
-    (guix port host nar-path cache workers ttl)
+    (guix port host nar-path cache workers ttl cache-bypass-threshold)
     (list (shepherd-service
            (provision '(guix-publish))
            (requirement '(guix-daemon))
@@ -1796,7 +1798,11 @@ raise a deprecation warning if the 'compression-level' field was used."
                                                     "s"))
                                   #~())
                            #$@(if cache
-                                  #~((string-append "--cache=" #$cache))
+                                  #~((string-append "--cache=" #$cache)
+                                     #$(string-append
+                                        "--cache-bypass-threshold="
+                                        (number->string
+                                         cache-bypass-threshold)))
                                   #~()))
 
                      ;; Make sure we run in a UTF-8 locale so we can produce