Avoid exporting variables in build scripts

Environment variables "pollute" the build environment of packages and
can affect their output.

This change results in the removal of some files from packages that
were not meant to be packaged. It also removes the need for a
workaround in automake 1.10.3 to manually remove such files.

Variables are now saved in an .env file for each system and included
in scripts that need them using the dot operation.

sysb/init

@@ -7,9 +7,10 @@
 
 set -e
 
-export PATH=/usr/bin
-export PREFIX=/usr
-export SOURCES=/usr/src
+PREFIX=/usr
+SOURCES="${PREFIX}/src"
+
+export PATH="${PREFIX}/bin"
 
 # shellcheck source=sysa/helpers.sh
 . /usr/src/helpers.sh
@@ -34,5 +35,10 @@ install_tar kexec-tools-2.0.22 0
 install_tar util-linux-2.19.1 0
 
 # Begin sysb bootstrapping process
-cd /usr/src
-./run.sh
+cd "${SOURCES}"
+
+# shellcheck disable=SC2129
+echo "PREFIX=${PREFIX}" >> .env
+echo "SOURCES=${SOURCES}" >> .env
+
+exec ./run.sh