allow use of env placeholder in tls certificates

[acuteaura]

I'm using this directive to load certificates through systemd's LoadCredential= (reference here).

        tls "{env.CREDENTIALS_DIRECTORY}/cert.pem" "{env.CREDENTIALS_DIRECTORY}/cert.key"

Unfortunately, the placeholder does not seem to get resolved.

Dec 04 10:49:00 caddy[1517810]: Error: loading http app module: provision http: getting tls app: loading tls app module: provision tls: loading certificates: open {env.CREDENTIALS_DIRECTORY}/cert.pem: no such file or directory

[francislavoie]

FYI you can use {$ENV} syntax instead for now, since that's replaces at config-adapt time instead of at runtime, so no special support is needed for that to work. https://caddyserver.com/docs/caddyfile/concepts#environment-variables

But either way, no harm in adding this, and @mohammed90 already got to it quickly 😊 #5963