* guix/scripts/substitute-binary.scm (fetch): In the `file' case, open
with the `b' flag, so that the coding cookie reading thing doesn't
lead to buffering some of the data (on 2.0.5).
* tests/utils.scm ("filtered-port, file"): Open with `r0b'. Fixes a
test failure with Guile 2.0.5 whereby the first byte of FILE would be
missing from DECOMPRESSED.