cmd/go: go build either looks for go.mod in the directory specified by -C or doesn't depending on where -C appears relative to -gcflags

[aarzilli]

$ go version
go version devel go1.21-40bdc56180 Tue May 23 13:22:25 2023 +0000 linux/amd64

but also reproduces with 1.20. Reproducer:

$ mkdir example
$ cd example
$ go mod init example
$ cat - > main.go
package main
func main() { println("hello world") }
^D
$ cd ..
$ go build -C example -gcflags=""
(succeeds)
$ go build -gcflags="" -C example
go: go.mod file not found in current directory or any parent directory; see 'go help modules'

Originally reported at go-delve/delve#3380 (comment)

[aarzilli]

cc @bcmills

[seankhliao]

both -C and the build flags are processed as they are set, doesn't seem like an easy fix

go/src/cmd/go/internal/base/flag.go

Line 65 in 7ad92e9

flags.Func("C", "AddChdirFlag", os.Chdir)

go/src/cmd/go/internal/work/build.go

Line 313 in 7ad92e9

cmd.Flag.Var(&load.BuildGcflags, "gcflags", "")

go/src/cmd/go/internal/load/flag.go

Line 38 in 7ad92e9

return f.set(v, base.Cwd())

[bcmills]

The documentation for go build currently states:

If used, [the -C] flag must be the first one in the command line.

That restriction was added in https://go.dev/cl/500915 for #57001.

Closing as probably fixed, but please let us know if you can still reproduce similar problems using Go 1.21 or higher.