Zcu: fix ZOIR cache bugs

* When saving bigint limbs, we gave the iovec the wrong length, meaning
  bigint data (and the following string and compile error data) was corrupted.
* When updating a stale ZOIR cache, we failed to truncate the file, so
  just wrote more bytes onto the end of the stale cache.

src/Zcu.zig

@@ -2753,7 +2753,7 @@ pub fn saveZoirCache(cache_file: std.fs.File, stat: std.fs.File.Stat, zoir: Zoir
         },
         .{
             .base = @ptrCast(zoir.limbs),
-            .len = zoir.limbs.len * 4,
+            .len = zoir.limbs.len * @sizeOf(std.math.big.Limb),
         },
         .{
             .base = zoir.string_bytes.ptr,

src/Zcu/PerThread.zig

@@ -234,6 +234,7 @@ pub fn updateFile(
             error.FileTooBig => unreachable, // 0 is not too big
             else => |e| return e,
         };
+        try cache_file.seekTo(0);
 
         if (stat.size > std.math.maxInt(u32))
             return error.FileTooBig;