Explicitly call rspec executable from srcdir #759

Job	Run time
make (check, cppflags=-DVM_CHECK_MODE)	7m 53s
make (check, i686)	10m 54s
make (check, --disable-yjit)	6m 50s
make (check, --enable-shared --enable-load-relative)	7m 34s
make (check, true, --with-shared-gc=/home/runner/ruby_gc)	7m 19s
make (test-bundler-parallel, 50)	27m 14s
make (test-bundled-gems)	3m 51s
make (check, ubuntu-20.04)	9m 4s
make (check, ubuntu-24.04)	8m 58s
Ubuntu result	1s
	1h 29m 38s

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