Investigate I/O error when caching buildpack downloads

[fbiville]

Sometimes, the cached buildpack ($HOME/.pack/dl-cache/$sha256/buildpack.toml) gets corrupted with an extra ], leading to errors like this:

ERROR: failed to generate layer for buildpack io.projectriff.riff: reading buildpack.toml from buildpack: /Users/fbiville/.pack/dl-cache/50d30c4c48f8533f9ddbfa2dafc539b49892469417e0883adb0eb0ed1f49d675: Near line 61 (last key parsed 'metadata'): bare keys cannot contain ']'
make: *** [build] Error 1

This is symptomatic of an I/O error in Golang (e.g. resource not closed).