Base: PCRE: exec: type assert String after construction (#57868)

Should prevent some invalidation in the sysimage.

(cherry picked from commit f49f46d)

Author: nsajko

base/pcre.jl

@@ -199,7 +199,7 @@ end
 exec(re, subject::Union{String,SubString{String}}, offset, options, match_data) =
     _exec(re, subject, offset, options, match_data)
 exec(re, subject, offset, options, match_data) =
-    _exec(re, String(subject), offset, options, match_data)
+    _exec(re, String(subject)::String, offset, options, match_data)
 
 function _exec(re, subject, offset, options, match_data)
     rc = ccall((:pcre2_match_8, PCRE_LIB), Cint,