This reverts commit 5da66561d1.
It seems the chromium build now unconditionally tries to enable ozone
(even though we disable it), causing the build to fail (as we only
provide xkbcommon when enabling Ozone):
```
configuring
ERROR at //build/config/linux/pkg_config.gni:103:17: Script returned non-zero exit code.
pkgresult = exec_script(pkg_config_script, args, "value")
^----------
Current dir: /build/chromium-87.0.4252.0/out/Release/
Command: python /build/chromium-87.0.4252.0/build/config/linux/pkg-config.py xkbcommon
Returned 1.
stderr:
Package xkbcommon was not found in the pkg-config search path.
Perhaps you should add the directory containing `xkbcommon.pc'
to the PKG_CONFIG_PATH environment variable
No package 'xkbcommon' found
Could not run pkg-config.
See //ui/events/ozone/layout/BUILD.gn:12:3: whence it was called.
pkg_config("xkbcommon") {
^------------------------
See //chrome/test/chromedriver/BUILD.gn:273:15: which caused the file to be included.
deps += [ "//ui/events/ozone/layout" ]
^-------------------------
builder for '/nix/store/2dqhrd2qzyms078wnvwv6ays53ppvgc2-chromium-unwrapped-87.0.4252.0.drv' failed with exit code 1
cannot build derivation '/nix/store/4iyhgzsmpx80v75hvk1jycwzanw4z5dn-chromium-dev-87.0.4252.0.drv': 1 dependencies couldn't be built
```
update.nix was a huuuuge hack, abusing checksum collisions, etc., and
was extremely difficult to read and maintain, especially because
values from update.nix were also used in the derivations themselves!
I've replaced this with an implementation in Python, which I chose for
readability. Rather than generating Nix, I chose to
generate JSON, since Python can do that in the standard library and
Nix can read it.
I also set update.py as an updateScript, so Chromium can now
automatically be updated!
Fixes: https://github.com/NixOS/nixpkgs/issues/89635
