Support reproducible builds (#1802)

Support reproducible builds (#1802)

* Makefile: Ensure we strip out embedded paths

To reproduce binaries undeterministic values needs to be removed. By
default Go embeds several module paths into the binaries, which prevents
people from reproducing said distributed binary.

The distributed binary from micro contains the full home path of the
current builder of the binary. -trimpath removes these paths from the
binary.

    $ strings micro | grep "/home/zyedidia" | wc -l
    868

This also helps other distributions providing reproducible versions of
micro down the line.

Signed-off-by: Morten Linderud <morten@linderud.pw>
* build-date: Ensure build time adheres to SOURCE_DATE_EPOCH

Embedding undeterministic values into binaries prevents reproduction of
the binaries. The reproducible builds projects defines
`SOURCE_DATE_EPOCH` to allow deterministic insertion of build times.

This patch ensures `build-date` checks the environment variable before
building with the local time.

    $ SOURCE_DATE_EPOCH=123123 go run tools/build-date.go
    January 02, 1970
    $ go run tools/build-date.go
    July 31, 2020

    $ make build-quick && ./micro --version
    [...]
    Compiled on July 31, 2020
    $ SOURCE_DATE_EPOCH=123123 make build-quick && ./micro --version
    [...]
    Compiled on January 02, 1970

https://reproducible-builds.org/specs/source-date-epoch/

Signed-off-by: Morten Linderud <morten@linderud.pw>