rely on go build stamping a version for local builds

Before Go 1.24, `go build` only stamped module versions for modules
resolved via GOPROXY, as the local module only had VCS information.
For that reason, we manually built a pseudo-version from the VCS
timestamp and revision stamped for local builds.

Go 1.24 started stamping the main module with a module version
derived from the local VCS information, much like we already did.
For example, comparing a clean build before this change
against a build with this uncommitted change:

    $ go install
    $ garble version
    mvdan.cc/garble v0.14.3-0.20250413182748-e97968ccae46
    [...]
    $ git stash pop
    $ go install
    $ garble version
    mvdan.cc/garble v0.14.3-0.20250413182748-e97968ccae46+dirty
    [...]

The only user-visible change is that local builds with any
uncommitted changes now get a `+dirty` suffix, but that's probably
a good thing for the majority of users, and provides a useful hint
in case a user forgot about local changes.

The test logic to inject VCS information via an env var
and see that the resulting pseudo-version is what we expect can go too,
as that was testing our own main module version logic.
We now rely on `go build` to do the right thing, so don't test that.

testdata/script/help.txtar

@@ -87,26 +87,12 @@ stderr 'directory not found'
 
 # Test the version command. Note that test binaries exclude VCS build info,
 # and we reuse the test binary for garble itself, so that's missing.
-# To avoid building another garble binary,
-# and to be able to use static VCS info, use an environment variable.
-# First, test without the information, and then with it.
 exec garble version
 stdout -count=1 'mvdan.cc/garble \(devel\)'
 stdout -count=1 'Build settings'
 stdout -count=3 '-compiler|GOOS|GOARCH'
 ! stdout 'vcs'
 
-# Obtained from a real build while developing.
-env GARBLE_TEST_BUILDSETTINGS='[{"Key":"vcs","Value":"git"},{"Key":"vcs.revision","Value":"91ea246349544769f5100c29f79cb0f173abfeea"},{"Key":"vcs.time","Value":"2022-03-18T13:45:11Z"},{"Key":"vcs.modified","Value":"true"}]'
-exec garble version
-stdout -count=1 'mvdan\.cc/garble v0\.0\.0-20220318134511-91ea24634954'
-stdout -count=1 'Build settings'
-stdout -count=3 '-compiler|GOOS|GOARCH'
-stdout -count=1 'vcs git'
-stdout -count=1 'vcs\.revision 91ea246349544769f5100c29f79cb0f173abfeea'
-stdout -count=1 'vcs\.time 2022-03-18T13:45:11Z'
-stdout -count=1 'vcs\.modified true'
-
 ! exec garble version -flag
 stderr 'usage: garble version'