Clear timezones from cached zip entries

In https://gitlab.com/gitlab-org/gitlab-pages/-/issues/702 we
discovered that by caching many `*zip.File` instances, we end up
storing many Go timezone objects on the heap: one per cached
`*zip.File`. This adds up to about 25% of the heap size.

In this commit we set the timestamp of `*zip.File` to UTC. Because Go
re-uses a single timezone object for UTC, this causes the original
timezone objects of the `*zip.File` instances to be garbage collected.
This reduces the heap size.

Here is an example program that demonstrates the effect:

```golang
package main

import (
	"archive/zip"
	"log"
	"os"
	"runtime/pprof"
)

func main() {
	if err := load(os.Args[1]); err != nil {
		log.Fatal(err)
	}
	log.Printf("load finished: %d archives", len(readers))
	pprof.WriteHeapProfile(os.Stdout)
}

var readers []*zip.ReadCloser

func load(filename string) error {
	for i := 0; i < 1000; i++ {
		zr, err := zip.OpenReader(filename)
		if err != nil {
			return err
		}
		if os.Getenv("FORCE_UTC") == "1" {
			for _, zf := range zr.File {
				zf.Modified = zf.Modified.UTC()
			}
		}

		readers = append(readers, zr)
	}

	return nil
}
```

1 files changed, 7 insertions, 0 deletions

diff --git a/internal/vfs/zip/archive.go b/internal/vfs/zip/archive.go
index 0a9ac0d7..49ae2719 100644
--- a/internal/vfs/zip/archive.go
+++ b/internal/vfs/zip/archive.go
@@ -148,6 +148,13 @@ func (a *zipArchive) readArchive(url string) {
 			continue
 		}
 
+		// Each Modified timestamp contains a pointer to a unique timezone
+		// object. This wastes a lot of memory. By setting the timezone to UTC on
+		// each timestamp, we allow the unique timezone objects to be
+		// garbage-collected. Also see
+		// https://gitlab.com/gitlab-org/gitlab-pages/-/issues/702.
+		file.Modified = file.Modified.UTC()
+
 		if file.Mode().IsDir() {
 			a.directories[file.Name] = &file.FileHeader
 		} else {