Allow using preprocessor mode from env without a disk config

[Alphare]

If we set SCCACHE_DIRECT=true but not disk_dir or disk_sz, we end up with None which sets use_preprocessor_cache_mode to false.

But if we run:

./target/debug/sccache --start-server
./target/debug/sccache --show-stats

We can see that it is uses the local disk by default.

After this change, this works:
SCCACHE_DIRECT=true ./target/debug/sccache --start-server ./target/debug/sccache --show-stats

[Alphare]

@sylvestre this should fix #1941 but highlighted a deeper issue with the config system.

IIUC the config from env is merged with the config file's at a coarse granularity (like azure, disk, etc.).
The user can then get into a situation where they've defined a disk_dir and disk_sz in the config file, but only set use_preprocessor_cache_mode with the environment variable: this means that the two first settings will be ignored by sccache.

Is this in scope for a fix? I think it might be a bit involved since we'd need to define the merge logic either manually for every field (which is brittle, not future-proof and prone to errors), or use some form of reflection to loop through the fields.

[sylvestre]

looks like you didn't rebase :)

[sylvestre]

if you have time, yeah, it would be nice to add some extra checks