back.rtlil: espace { and } in format string #3568

Job	Run time
check-links	46s
document	43s
smoke (amaranth-lang/amaranth-boards)	33s
test (3.8)	2m 36s
smoke (amaranth-lang/amaranth-stdio)	36s
test (3.9)	2m 22s
smoke (amaranth-lang/amaranth-soc)	44s
test (3.10)	2m 31s
test (3.11)	2m 57s
test (3.12)	3m 43s
test (pypy-3.8)	5m 17s
test (pypy-3.9)	5m 16s
test (pypy-3.10)	5m 11s
test (3.13-dev)	3m 29s
publish-docs	6s
publish-docs-dev	0s
required	1s
publish-schemas	5s
publish-package	41s
publish-release	0s
	37m 37s

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