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Run pipeline in background from git hook

I'm trying to run a pipeline to update a PDF after every push to the 'main' branch.  I want it to be atomic, so it doesn't touch the existing PDF until it has finished, so I need to use sponge(1) (from moreutils).

I've tried the following script:

```sh
#!/bin/sh

test "$1" = "refs/heads/main" || exit 0;

cd ~/tmp/man-pages/;

unset $(git rev-parse --local-env-vars);
git fetch srv			>/dev/null;
git reset srv/main --hard	>/dev/null;

do_work()
{
	scripts/LinuxManBook/build.sh 2>/dev/null \
	| sponge ~/tmp/LMB-HEAD.pdf >/dev/null 2>&1;
}

do_work &
```

If I run the script in a terminal, it works as expected:

```sh
$ time ./hooks/post-update refs/heads/main

real	0m0.015s
user	0m0.007s
sys	0m0.008s
```

And after 15 seconds, the PDF is there.

However, if I do a git push, I must wait for it to finish.  It seems to be ignoring the `&`.

```sh
$ time git push -f tmp 
Enumerating objects: 8, done.
Counting objects: 100% (8/8), done.
Delta compression using up to 24 threads
Compressing objects: 100% (5/5), done.
Writing objects: 100% (5/5), 1.40 KiB | 1.40 MiB/s, done.
Total 5 (delta 3), reused 0 (delta 0), pack-reused 0
remote: From /home/alx/tmp/man
remote:  + 3ac7ec68b...8b824c494 main       -> srv/main  (forced update)
To /home/alx/tmp/man.git/
 + 3ac7ec68b...8b824c494 main -> main (forced update)

real	0m16.051s
user	0m0.025s
sys	0m0.014s
```

Is there anything I'm missing?  If I transform the script to do a simpler redirection, without using a pipe, then it works as expected.  It seems it's the pipe that is making me wait.

```sh
...

scripts/LinuxManBook/build.sh >~/tmp/LMB-HEAD.pdf &  #This works
```