Prevent jinja `let` to move variables #78

Job	Run time
Audit	22s
MSRV	1m 47s
Lint	17s
Package (rinja)	30s
Package (rinja_actix)	52s
Test (ubuntu-latest, stable)	38s
Package (rinja_axum)	30s
Test (ubuntu-latest, beta)	46s
Package (rinja_derive)	19s
Test (macos-latest, stable)	40s
Package (rinja_derive_standalone)	42s
Test (windows-latest, stable)	2m 46s
Package (rinja_escape)	29s
Package (rinja_parser)	32s
Package (rinja_rocket)	1m 1s
Package (rinja_warp)	40s
Package (testing)	1m 3s
	13m 54s

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