Change shell preamble fron /bin/bash to /usr/bin/sh #115

Job	Run time
build (clang)	6m 37s
build (clang, CFLAGS=-flto LDFLAGS=-flto)	11m 50s
build (gcc)	6m 5s
build (gcc, CFLAGS=-flto LDFLAGS=-flto)	6m 56s
build (gcc-9, NO_32BIT_TEST=1 AR=gcc-ar-9 NM=gcc-nm-9 RANLIB=gcc-ranlib-9 )	5m 50s
build (gcc-10, NO_32BIT_TEST=1 AR=gcc-ar-10 NM=gcc-nm-10 RANLIB=gcc-ranlib-10 )	5m 58s
build (gcc-10, NO_32BIT_TEST=1 AR=gcc-ar-10 NM=gcc-nm-10 RANLIB=gcc-ranlib-10 CFLAGS=-flto LDFLAG...	12m 29s
build (gcc-12, NO_32BIT_TEST=1 AR=gcc-ar-12 NM=gcc-nm-12 RANLIB=gcc-ranlib-12 )	6m 2s
build (gcc-12, NO_32BIT_TEST=1 AR=gcc-ar-12 NM=gcc-nm-12 RANLIB=gcc-ranlib-12 CFLAGS=-flto LDFLAG...	5m 15s
	1h 7m 2s

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