sway/config.c: use memcpy() for known buffer size

`gcc-14` added a new warning around dangerous use of `strncpy()` withi
known overflow:

    ../sway/config.c: In function 'do_var_replacement':
    ../sway/config.c:983:33: error: '__builtin___strncpy_chk' specified bound depends on the length of the source argument [-Werror=stringop-truncation]
      983 |                                 strncpy(newptr, var->value, vvlen);
          |                                 ^
    ../sway/config.c:971:45: note: length computed here
      971 |                                 int vvlen = strlen(var->value);
          |                                             ^~~~~~~~~~~~~~~~~~

It's a bit fishy to rely on truncating behaviour of `strncpy()`. The
change uses `memcpy()` as more explicit way to express copy of `vvlen`
bytes.

(cherry picked from commit 2cd73a33c2)

sway/config.c

@@ -975,7 +975,7 @@ char *do_var_replacement(char *str) {
 				int offset = find - str;
 				strncpy(newptr, str, offset);
 				newptr += offset;
-				strncpy(newptr, var->value, vvlen);
+				memcpy(newptr, var->value, vvlen);
 				newptr += vvlen;
 				strcpy(newptr, find + vnlen);
 				free(str);