add -d:nimStrictMode in CI to keep code from regressing; fixes ConvFromXtoItselfNotNeeded, UnusedImport notes (#16764)

lib/system/dollars.nim

@@ -5,7 +5,6 @@ proc `$`*(x: int): string {.magic: "IntToStr", noSideEffect.}
 
 when defined(js):
   import std/private/since
-
   since (1, 3):
     proc `$`*(x: uint): string =
       ## Caveat: currently implemented as $(cast[int](x)), tied to current

lib/system/excpt.nim

@@ -378,7 +378,8 @@ proc reportUnhandledErrorAux(e: ref Exception) {.nodestroy.} =
     # ugly, but avoids heap allocations :-)
     template xadd(buf, s, slen) =
       if L + slen < high(buf):
-        copyMem(addr(buf[L]), cstring(s), slen)
+
+        copyMem(addr(buf[L]), (when s is cstring: s else: cstring(s)), slen)
         inc L, slen
     template add(buf, s) =
       xadd(buf, s, s.len)