Ensure DefaultEnvVariables is used in Specgen

When we rewrote Podman's pkg/spec, one of the things that was
lost was our use of a set of default environment variables, that
ensure all containers have at least $PATH and $TERM set.

While we're in the process of re-adding it, change it from a
variable to a function, so we can ensure the Join function does
not overwrite it and corrupt the defaults.

Signed-off-by: Matthew Heon <matthew.heon@pm.me>

pkg/env/env.go

@@ -12,14 +12,16 @@ import (
 	"github.com/pkg/errors"
 )
 
-// DefaultEnvVariables sets $PATH and $TERM.
-var DefaultEnvVariables = map[string]string{
-	"PATH": "/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin",
-	"TERM": "xterm",
-}
-
 const whiteSpaces = " \t"
 
+// DefaultEnvVariables returns a default environment, with $PATH and $TERM set.
+func DefaultEnvVariables() map[string]string {
+	return map[string]string{
+		"PATH": "/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin",
+		"TERM": "xterm",
+	}
+}
+
 // Slice transforms the specified map of environment variables into a
 // slice. If a value is non-empty, the key and value are joined with '='.
 func Slice(m map[string]string) []string {