cmake/FindRust.cmake: inherit RUSTFLAGS from the environment #2453

Job	Run time
check (libclamav, (iana_cctld\|bytecode_api_\|bytecode_hooks\|rijndael\|yara\|inffixed\|inflate\|queue\|n...	10s
check (libfreshclam)	13s
check (clamav-milter)	10s
check (clambc)	8s
check (clamconf)	9s
check (clamd)	14s
check (clamdscan)	12s
check (clamdtop)	10s
check (clamonacc, (c-thread-pool\|fts\|priv_fts))	19s
check (clamscan)	16s
check (clamsubmit)	9s
check (freshclam)	10s
check (libfreshclam)	13s
check (common)	19s
check (sigtool)	9s
check (examples)	15s
check (win32/compat)	18s
	3m 34s

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