Make fmt-check check what fmt writes (closes #22)

script/fmt writes with gofmt -s and goimports, but script/fmt-check
only ran gofmt -l. A file that gofmt -s would simplify, or whose
imports goimports would regroup, passed make check and then changed
on the next make fmt.

script/fmt-check now runs gofmt -s -l and goimports -l, prints each
file either would change once, and fails. It puts Go's bin directory
on PATH first, as script/fmt does, so it runs the goimports that
script/bootstrap installs. The README line on what make check runs
now names both formatters.

Model: opus-5-5

README.md

@@ -97,7 +97,7 @@ Future improvements under consideration:
 * Issues / PRs: <https://git.eeqj.de/sneak/attrsum/>
 * Code must pass `make check`, which runs the tests and golangci-lint as
   phases of the `Dockerfile` (Docker is required) and checks formatting
-  with `gofmt`.
+  with `gofmt -s` and goimports.
 * No CLA; contributions are under WTFPL v2.
 
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