Report the Go build info version when linker flags set none (closes #422)

A `go install` of a tag sets no linker flags, so the binary reported
`dev`. Version, commit and commit date that the linker flags leave
unset now come from the build info Go records. Go records a
pseudo-version for an untagged commit and a `+dirty` suffix for a
modified checkout; `IsDevVersion` now counts both as development
builds, using `golang.org/x/mod`.

Model: opus-5-5

script/version

@@ -11,8 +11,8 @@
 # when no tag is reachable, each with a "-dirty" suffix when tracked
 # files have uncommitted changes. Untracked files are ignored: a stray
 # scratch file does not change what was compiled. Outside a git checkout
-# (release tarball, `go install`), or in one with no commits, it is
-# "dev".
+# (such as an unpacked release tarball), or in one with no commits, it
+# is "dev".
 #
 # Nothing here ever invents a version number.
 set -eu