Fixes cache bug

Author: MasonRemaley

src/Zcu.zig

@@ -2748,7 +2748,7 @@ 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;