tools/e2fsprogs: fixup STAGING_DIR_HOST path evaluation in shell scripts

tools/e2fsprogs: fixup STAGING_DIR_HOST path evaluation in shell scripts

We have to use curly braces on the exported STAGING_DIR_HOST env variable,
instead of evaluating it directly as we are not in Make but a separate
shell script.

Otherwise it would fail with:
staging_dir/host/bin/compile_et: line 6: STAGING_DIR_HOST: command not found
staging_dir/host/bin/mk_cmds: line 5: STAGING_DIR_HOST: command not found

And so when krb5 tries to build it will fail as compile_et and mk_cmds will
return an error.

Fixes: 55bda9863dd0 ("tools/e2fsprogs: fix shell scripts under SDK")
Link: https://github.com/openwrt/openwrt/pull/15854
Signed-off-by: Robert Marko <robimarko@gmail.com>