scripts/auto-bootstraps/dobootstrap: unbreak after shellcheck

the quoting-eagerness of shellcheck isn't always resulting in the
desired output, in this case quoted empty strings broke where they
previously would be ignored as whitespace separators

Signed-off-by: Fabian Groffen <[email protected]>

Author: grobian

scripts/auto-bootstraps/dobootstrap

@@ -127,20 +127,20 @@ do_prepare() {
 	starttime=${SECONDS}
 	env -i \
 		HOME="${EPREFIX}" \
-		SHELL=/bin/sh \
+		SHELL="/bin/sh" \
 		TERM="${TERM}" \
 		USER="${USER}" \
 		CHOST="${chost}" \
 		EPREFIX="${EPREFIX}" \
-		${GENTOO_MIRRORS+GENTOO_MIRRORS="${GENTOO_MIRRORS}"} \
+		${GENTOO_MIRRORS+GENTOO_MIRRORS=\""${GENTOO_MIRRORS}"\"} \
 		${DOLOCAL+DOLOCAL=1} \
 		${RESUME+RESUME=1} \
 		${LATEST_TREE_YES+LATEST_TREE_YES=1} \
-		${TREE_FROM_SRC+TREE_FROM_SRC=}"${TREE_FROM_SRC}" \
-		${USE_CPU_CORES+USE_CPU_CORES=}"${USE_CPU_CORES}" \
-		${DARWIN_USE_GCC+DARWIN_USE_GCC=}"${DARWIN_USE_GCC}" \
-		${PREFIX_DISABLE_RAP+PREFIX_DISABLE_RAP=}"${PREFIX_DISABLE_RAP}" \
-		${BP_KEEPALIVE_ACTIVE+BP_KEEPALIVE_ACTIVE=}"${BP_KEEPALIVE_ACTIVE}" \
+		${TREE_FROM_SRC+TREE_FROM_SRC="${TREE_FROM_SRC}"} \
+		${USE_CPU_CORES+USE_CPU_CORES="${USE_CPU_CORES}"} \
+		${DARWIN_USE_GCC+DARWIN_USE_GCC="${DARWIN_USE_GCC}"} \
+		${PREFIX_DISABLE_RAP+PREFIX_DISABLE_RAP="${PREFIX_DISABLE_RAP}"} \
+		${BP_KEEPALIVE_ACTIVE+BP_KEEPALIVE_ACTIVE="${BP_KEEPALIVE_ACTIVE}"} \
 		"${BASH}" "${bootstrapscript}" bootstrap
 	endtime=${SECONDS}