fix(libmain/common-args): do not exceed maximum allowed verbosity

This patch gets rid of UB when verbosity exceeds the maximum logging value of `lvlVomit = 7` and
reaches invalid values (e.g. 8). This is actually triggered in functional tests.
There are too many occurrences to list, but here's one from the UBSAN log:

../src/libstore/gc.cc:610:5: runtime error: load of value 8, which is not a valid value for type 'Verbosity'

src/libmain/common-args.cc

@@ -17,7 +17,9 @@ MixCommonArgs::MixCommonArgs(const std::string & programName)
         .shortName = 'v',
         .description = "Increase the logging verbosity level.",
         .category = loggingCategory,
-        .handler = {[]() { verbosity = (Verbosity) (verbosity + 1); }},
+        .handler = {[]() {
+            verbosity = (Verbosity) std::min<std::underlying_type_t<Verbosity>>(verbosity + 1, lvlVomit);
+        }},
     });
 
     addFlag({