Truncate .env file when writing to it

This makes no functional difference in practice, but is a bit cleaner
in case init scripts are executed multiple times (for testing or
otherwise).

sysb/init

@@ -37,8 +37,7 @@ install_tar util-linux-2.19.1 0
 # Begin sysb bootstrapping process
 cd "${SOURCES}"
 
-# shellcheck disable=SC2129
-echo "PREFIX=${PREFIX}" >> .env
+echo "PREFIX=${PREFIX}" > .env
 echo "SOURCES=${SOURCES}" >> .env
 
 exec ./run.sh

sysc/init

@@ -63,8 +63,8 @@ ln -s /usr/bin/bash /usr/bin/sh
 # Begin sysc bootstrapping process
 cd "${SOURCES}"
 
+echo "PREFIX=${PREFIX}" > .env
 # shellcheck disable=SC2129
-echo "PREFIX=${PREFIX}" >> .env
 echo "SOURCES=${SOURCES}" >> .env
 echo "DESTDIR=${DESTDIR}" >> .env
 echo "DISTFILES=${DISTFILES}" >> .env