[plan-build.sh] Fix a `last_build.env` formatting regression.
[plan-build.sh] Fix a `last_build.env` formatting regression.

This restores the format of the `./results/last_build.env` file's
`pkg_blake2bsum` line. Some speed improvements and refactoring in #1804
resulted in `hab pkg hash` to now output the checksum *and* filename
rather than simply the checksum. This fix only keeps the first token in
the command's result string--that is, the checksum only.

Signed-off-by: Fletcher Nichol <77a0fd9e8048bbacd11af4e957bc6ff03b549f49@nichol.ca>

habitat-sh/habitat,habitat-sh/habitat,georgemarshall/habitat,nathenharvey/habitat,rsertelon/habitat,habitat-sh/habitat,georgemarshall/habitat,habitat-sh/habitat,georgemarshall/habitat,rsertelon/habitat,rsertelon/habitat,georgemarshall/habitat,georgemarshall/habitat,nathenharvey/habitat,georgemarshall/habitat,rsertelon/habitat,rsertelon/habitat,habitat-sh/habitat,habitat-sh/habitat,habitat-sh/habitat,georgemarshall/habitat,georgemarshall/habitat,nathenharvey/habitat,rsertelon/habitat,nathenharvey/habitat,rsertelon/habitat,nathenharvey/habitat,habitat-sh/habitat,nathenharvey/habitat