feat: add .env.markflow configuration and update wiki workflow to use it #15

Job	Run time
runtime smoke	19s
SIMD contract (Cranelift)	18s
parse conformance	23s
type + lowering conformance	20s
backend features (default)	25s
fmt + clippy	17s
backend features (llvm-backend)	1m 14s
	3m 16s

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