bug#27437: Source downloader accepts X.509 certificate for incorrect domain

[Leo Famulari <leo@famulari.name>]

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While working on some package updates, I found that the source code
downloader will accept an X.509 certificate for an incorrect site.

Here is what happens:

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$ ./pre-inst-env guix build -S opus-tools --check
@ build-started /gnu/store/nn93hkik8kvrigcf2pvmym01zg7jqm4v-opus-tools-0.1.10.tar.gz.drv - x86_64-linux /var/log/guix/drvs/nn//93hkik8kvrigcf2pvmym01zg7jqm4v-opus-tools-0.1.10.tar.gz.drv.bz2
 
Starting download of /gnu/store/0js62s7pz9gfcdsd1n764w91mhhwkws4-opus-tools-0.1.10.tar.gz
From https://downloads.xiph.org/releases/opus/opus-tools-0.1.10.tar.gz...
 ….1.10.tar.gz  305KiB              822KiB/s 00:00 [####################] 100.0%
warning: rewriting hashes in `/gnu/store/vdpyfqzp0kkjpxr79fq3an7j4s4vkz0h-opus-tools-0.1.10.tar.gz'; cross fingers
/gnu/store/vdpyfqzp0kkjpxr79fq3an7j4s4vkz0h-opus-tools-0.1.10.tar.gz
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Here is an example of what I think should happen in this case:

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$ curl https://downloads.xiph.org/releases/opus/opus-tools-0.1.10.tar.gz
curl: (51) SSL: certificate subject name (osuosl.org) does not match target host name 'downloads.xiph.org'
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And this is what Firefox says:

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downloads.xiph.org uses an invalid security certificate.

The certificate is only valid for the following names:
  osuosl.org, *.osuosl.org  

Error code: SSL_ERROR_BAD_CERT_DOMAIN
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