Fall back to the default when an environment variable cannot be read

[SonnyRR]

An environment variable the build cannot read now falls back to the default instead of aborting the process. The FALLOUT_ prefix documented in the parameters page now works.

What changed

• An unusable environment value reports a warning on standard error and uses the default for that parameter, instead of throwing. Built-in parameters resolve inside FalloutBuild's static constructor, so throwing there killed the process before any target ran.
• Warnings go to standard error rather than Serilog, because Serilog is not configured yet at that point. This also makes the existing "multiple values are provided" warning visible, which it never was.
• FALLOUT_ and NUKE_ prefixed names are matched before the bare parameter name.
• Documented the precedence and the new failure behaviour on the parameters page.

Why the prefixes are matched first

The bare match is case-insensitive, so on Unix the HOST variable matches the Host parameter. If the bare match ran first it would always shadow FALLOUT_HOST, and the documented prefix would have no effect on the parameter that needs it most.

This also fixes an explicit override being dropped. With both HOST and NUKE_HOST set, both were collected as candidates, the resolver saw more than one, and it fell back to the default.

No behaviour is removed

Every environment variable spelling that resolved before still resolves: Host=, host= in lower case, NUKE_HOST=, --host, and the bare fuzzy match. Nothing about matching was narrowed, so the change is identical on Windows, macOS and Linux.

One case changes: a FALLOUT_* variable that is set today and ignored will start taking effect. This is unavoidable, because HOST and Host are the same string under case-insensitive comparison, so a bare match would always shadow the prefix.

Verification

• Fallout.Build.Specs: 254 passed.
• Dogfooded against this repo's own build with HOST=M2, the case from [Bug] Standard Unix HOST environment variable hijacks the built-in Host parameter - every build crashes in the static constructor #705. Before: TypeInitializationException, exit 1. After: exit 0, one warning, help renders.
• HOST=M2 FALLOUT_HOST=Terminal produces no warning. The prefixed value wins over the bare HOST.
• Rebased onto develop after Move ParameterService onto the BuildContext #452 moved ParameterService. Re-ran the full suite and the same dogfood check afterwards.

Where this reproduces

Only where something exports HOST. On openSUSE the /etc/profile from aaa_base does it for every login shell. macOS does not export it, and neither do non-login shells in CI images or container entrypoints, which is likely why it has gone unnoticed. Details in #705.

The two commits are separable. The first fixes the crash on its own; the second adds the FALLOUT_ prefix and the precedence rule.

Closes #705
Fixes #457

[ChrisonSimtian]

lets look into the logger change please, we should avoid calling Console et al directly

[SonnyRR]

@ChrisonSimtian, I've addressed your feedback. Would appreciate a re-review when you have some spare time. :)

[ChrisonSimtian]

awesome thanks :-) lgtm

[dennisdoomen]

@SonnyRR please close all review comments.

[SonnyRR]

@SonnyRR please close all review comments.

Done.