Remove call to apt update before apt purge in the main doc build workflow

[regisss]

What does this PR do?

Fixes the current error in the main doc build workflow:

Get:1 file:/etc/apt/apt-mirrors.txt Mirrorlist [142 B]
Get:6 https://packages.microsoft.com/ubuntu/22.04/prod jammy InRelease
Err:6 https://packages.microsoft.com/ubuntu/22.04/prod jammy InRelease
  Clearsigned file isn't valid, got 'NOSPLIT' (does the network require authentication?)
Hit:2 http://azure.archive.ubuntu.com/ubuntu jammy InRelease
Get:3 http://azure.archive.ubuntu.com/ubuntu jammy-updates InRelease [119 kB]
Get:4 http://azure.archive.ubuntu.com/ubuntu jammy-backports InRelease [109 kB]
Get:5 http://azure.archive.ubuntu.com/ubuntu jammy-security InRelease [110 kB]
Get:7 http://azure.archive.ubuntu.com/ubuntu jammy-updates/main amd64 Packages [1610 kB]
Get:8 http://azure.archive.ubuntu.com/ubuntu jammy-updates/main Translation-en [303 kB]
Get:9 http://azure.archive.ubuntu.com/ubuntu jammy-updates/restricted amd64 Packages [1830 kB]
Hit:13 https://ppa.launchpadcontent.net/ubuntu-toolchain-r/test/ubuntu jammy InRelease
Get:10 http://azure.archive.ubuntu.com/ubuntu jammy-updates/restricted Translation-en [311 kB]
Get:11 http://azure.archive.ubuntu.com/ubuntu jammy-updates/universe amd64 Packages [1070 kB]
Get:12 http://azure.archive.ubuntu.com/ubuntu jammy-updates/universe Translation-en [244 kB]
Get:14 http://azure.archive.ubuntu.com/ubuntu jammy-backports/universe amd64 Packages [27.2 kB]
Get:15 http://azure.archive.ubuntu.com/ubuntu jammy-backports/universe Translation-en [16.2 kB]
Get:16 http://azure.archive.ubuntu.com/ubuntu jammy-security/main amd64 Packages [1392 kB]
Get:17 http://azure.archive.ubuntu.com/ubuntu jammy-security/main Translation-en [243 kB]
Get:18 http://azure.archive.ubuntu.com/ubuntu jammy-security/restricted amd64 Packages [1768 kB]
Get:19 http://azure.archive.ubuntu.com/ubuntu jammy-security/restricted Translation-en [299 kB]
Get:20 http://azure.archive.ubuntu.com/ubuntu jammy-security/universe amd64 Packages [848 kB]
Get:21 http://azure.archive.ubuntu.com/ubuntu jammy-security/universe Translation-en [1[63](https://github.com/huggingface/optimum/actions/runs/8813942295/job/24193513660#step:10:64) kB]
Reading package lists...
E: Failed to fetch https://packages.microsoft.com/ubuntu/22.04/prod/dists/jammy/InRelease  Clearsigned file isn't valid, got 'NOSPLIT' (does the network require authentication?)
E: The repository 'https://packages.microsoft.com/ubuntu/22.04/prod jammy InRelease' is no longer signed.
Error: Process completed with exit code 100.

For example here.

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
• Did you make sure to update the documentation with your changes?
• Did you write any new necessary tests?

Who can review?

[echarlaix]

Thanks for the fix @regisss !!