fix(gnovm): pin the per-file Go version in the consensus type-check

[davd-gzl]

Follow-up to #5893.

Whoever uploads a package decides which language rules it gets checked against. The chain should decide.

One comment line gets code accepted that should be rejected. for range 10 needs Go 1.22, which the VM cannot run:

//go:build go1.22

package z
func F() { for range 10 {} }   // rejected without the first line, accepted with it

The chain sets the language version to Go 1.18 before checking. That setting only applies to files which do not state a version themselves, and //go:build go1.N is how a file states one:

chain sets go1.18
   |
parse the file     the //go:build line sets the file's own version
   |
check the code     the file's version wins, so go1.22 is used

It also makes the answer depend on the machine. Go's type checker refuses a file asking for a version newer than the Go release used to build the node, so a package marked go1.26 is accepted by a node built with Go 1.26 and rejected by one built with Go 1.25:

file requires newer Go version go1.26 (application built with go1.25)

The fix erases the version a file states, right after parsing:

chain sets go1.18
   |
parse the file     the //go:build line sets the file's own version
   |
erase it           <- the fix
   |
check the code     nothing left to override, so go1.18 is used

A new test covers both cases and fails without the fix.

The second commit is unrelated cleanup: a nil check that can never run, left by the #5891 merge.

[Gno2D2]

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