Change shell preamble fron /bin/bash to /usr/bin/sh #114

Job	Run time
build (clang)	4m 47s
build (clang, CFLAGS=-flto LDFLAGS=-flto)	4m 50s
build (gcc)	5m 39s
build (gcc, CFLAGS=-flto LDFLAGS=-flto)	5m 24s
build (gcc-9, NO_32BIT_TEST=1 AR=gcc-ar-9 NM=gcc-nm-9 RANLIB=gcc-ranlib-9 )	4m 50s
build (gcc-10, NO_32BIT_TEST=1 AR=gcc-ar-10 NM=gcc-nm-10 RANLIB=gcc-ranlib-10 )	5m 44s
build (gcc-10, NO_32BIT_TEST=1 AR=gcc-ar-10 NM=gcc-nm-10 RANLIB=gcc-ranlib-10 CFLAGS=-flto LDFLAG...	5m 49s
build (gcc-12, NO_32BIT_TEST=1 AR=gcc-ar-12 NM=gcc-nm-12 RANLIB=gcc-ranlib-12 )	4m 54s
build (gcc-12, NO_32BIT_TEST=1 AR=gcc-ar-12 NM=gcc-nm-12 RANLIB=gcc-ranlib-12 CFLAGS=-flto LDFLAG...	5m 3s
	47m 0s

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