chore: use std.PrevRealm() in grc721 package #1650

Job	Run time
build (1.20.x, GOARCH=amd64 GOOS=linux, gno)	23s
build (1.20.x, GOARCH=wasm GOOS=js, gno)	34s
build (1.21.x, GOARCH=amd64 GOOS=linux, gno)	36s
test (1.20.x, _test.cmd)	1m 2s
test (1.20.x, _test.pkg)	49s
build (1.21.x, GOARCH=wasm GOOS=js, gno)	44s
test (1.20.x, _test.gnolang.native)	33s
test (1.20.x, _test.gnolang.stdlibs)	1m 2s
test (1.20.x, _test.gnolang.realm)	38s
test (1.20.x, _test.gnolang.pkg0)	1m 58s
test (1.20.x, _test.gnolang.pkg1)	9m 6s
test (1.20.x, _test.gnolang.pkg2)	5m 22s
test (1.20.x, _test.gnolang.other)	23s
test (1.21.x, _test.cmd)	1m 33s
test (1.21.x, _test.pkg)	1m 5s
test (1.21.x, _test.gnolang.native)	34s
test (1.21.x, _test.gnolang.stdlibs)	1m 5s
test (1.21.x, _test.gnolang.realm)	48s
test (1.21.x, _test.gnolang.pkg0)	2m 21s
test (1.21.x, _test.gnolang.pkg1)	7m 33s
test (1.21.x, _test.gnolang.pkg2)	5m 12s
test (1.21.x, _test.gnolang.other)	42s
	44m 3s

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