Use Existing Path Env Var for FNM (nushell#632)

The module for `fnm` did not set the path for Windows correctly. I fixed
it by searching for the used `Path` variable (or `PATH`) and use that
instead.

Works now better on Windows without breaking other OSes.

modules/fnm/fnm.nu

@@ -1,20 +1,22 @@
 export-env {
  def fnm-env [] {
  mut env_vars = {}
- let pwsh_vars = (^fnm env --shell power-shell)
- let var_table = ($pwsh_vars
- | lines
- | parse "$env:{key} = \"{value}\""
+ let pwsh_vars = (
+  ^fnm env --shell power-shell | 
+ lines | 
+ parse "$env:{key} = \"{value}\""
  )
- for v in $var_table {
- mut value: any = null
- if ($v.key | str downcase) == 'path' {
- $value = ($v.value | split row (char esep))
- } else {
- $value = $v.value
- }
- $env_vars = ($env_vars | insert $v.key $value)
+
+ # fnm-prefixed vars
+ for v in ($pwsh_vars | range 1..) { 
+ $env_vars = ($env_vars | insert $v.key $v.value) 
  }
+
+ # path
+ let env_used_path = ($env | columns | where {str downcase | $in == "path"} | get 0)
+ let path_value = ($pwsh_vars | get 0.value | split row (char esep))
+ $env_vars = ($env_vars | insert $env_used_path $path_value)
+
  return $env_vars
  }