fix(exec): deduplicate GOPATH before prepending version-specific path (#553)

When goenv exec is called from inside a shell that already has
GOPATH set by goenv (e.g. a tool like gup that shells out to
`go env GOPATH` through the goenv shim), exec.go was blindly
prepending the version-specific path without filtering existing
goenv-managed entries. This produced a doubled GOPATH such as:

  /Users/user/go/1.26.3:/Users/user/go/1.26.3

Which caused gup to construct the literal path
`/Users/user/go/1.26.3:/Users/user/go/1.26.3/bin` and fail
with "no such file or directory" when scanning for installed binaries.

Apply the same deduplication guard already present in sh-rehash.go:
filter out any $HOME/go/{version} entries from the existing GOPATH
before appending, so re-entrant exec calls stay idempotent.

Co-authored-by: Claude Sonnet 4.6 (1M context) <noreply@anthropic.com>

Author: jlec

cmd/shims/exec.go

@@ -125,8 +125,20 @@ func runExec(cmd *cobra.Command, args []string) error {
 			// This allows users to keep source code in existing locations while
 			// giving priority to version-specific installed tools/packages.
 			// See: https://github.com/go-nv/goenv/issues/147
+			// Filter out goenv-managed paths to prevent duplication when goenv exec
+			// is called from inside an already-managed shell (e.g. a tool that
+			// shells out to `go env GOPATH` through the shim).
 			if gopath != "" {
-				versionGopath = versionGopath + string(os.PathListSeparator) + gopath
+				goPathPattern := filepath.Join(homeDir, "go")
+				var filteredPaths []string
+				for _, p := range filepath.SplitList(gopath) {
+					if !strings.HasPrefix(p, goPathPattern+string(filepath.Separator)) || p == goPathPattern {
+						filteredPaths = append(filteredPaths, p)
+					}
+				}
+				if len(filteredPaths) > 0 {
+					versionGopath = versionGopath + string(os.PathListSeparator) + strings.Join(filteredPaths, string(os.PathListSeparator))
+				}
 			}
 
 			execEnv = setEnvVar(execEnv, utils.EnvVarGopath, versionGopath)