Production cert and key should support filesystem paths

[pi0]

Ref: https://github.com/nuxt/nuxt.js/issues/12827

Running a production server with SSL needs the NITRO_SSL_CERT and NITRO_SSL_KEY to be from filesystem.

As a workaround (UNIX and Mac), we can use:

NITRO_SSL_CERT="`cat path/to/cert.pem`" NITRO_SSL_KEY="`cat path/to/key.pem`" node .output/server/index.mjs