[bug#63877] Reworked patch for setting php-fpm environment variables

[guix]

Hi Bruno, sorry for taking a while to get back to this.

Writing a test for curl's behaviour with the SSL_CERT_DIR variable proved too
fiddly for me, so I gave up and wrote a simpler test that just checks for a
sentinel variable in the phpinfo output instead.

I also found out that php-fpm clears environment variables when it starts,
except for those listed in its configuration. However, libcurl isn't affected
by this as far as I can tell -- it needs the SSL_CERT_DIR variable to be set
in the process environment, not only in the php-fpm config file!

I decided to set environment variables in the process environment and list
them in the generated configuration file, so they're passed through to any PHP
programs run through PHP-FPM. This should minimise surprise, I hope.

(That's also be useful for setting e.g. PATH -- Nextcloud has started
complaining that that variable is unset, and it needs the variable to be
listed in the php-fpm configuration.)

The reworked patch also removes some of the gexp-related hairyness -- the
`environment-variables' property just takes a list of (variable-name . value)
pairs now, no gexp'ing required, though file-like objects like what
`file-append' returns are accepted.

Please let me know what you think, and thank you for your considerable
patience with this patch series! :)