fix(libexpr/eval-inline): get rid of references to nullptr env

[xokdvium]

Motivation

When diagnosing infinite recursion references to nullptr Env can be formed. This happens only with ExprBlackHole is evaluated, which always leads to InfiniteRecursionError.

UBSAN log for one such case:

../src/libexpr/eval-inline.hh:94:31: runtime error: reference binding to null pointer of type 'Env'
SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior ../src/libexpr/eval-inline.hh:94:31 in

Context

#10969

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[xokdvium]

// TBD: Maybe this case be handled directly without an idirection and a dummy
// environment (which isn't accesses anyway)?

I'm really not sure that this is the best way. This patch is pretty conservative, but I'm pretty sure that the only case when env is nullptr is for ExprBlackHole, since the assertion assert(env || v.isBlackhole()) holds in tests. I hope someone can shed some light on this.

If I'm correct in the assumption above, we could just move the code that throws InfiniteRecursionError to a member function that does not need an Env parameter and call it directly instead of eval with a dummy parameter. Since this is in the cold path and in the [[inlikely]] branch that throws perf should be of no concern hopefully.

[xokdvium]

Fixes #11854