Keep the mnemonic variables from the mnemonic command (closes #80)

The mnemonic command ran with the tool's whole environment, so a
mnemonic set in KEYFUNC_MNEMONIC reached it, although the command is
there to supply one of its own. It now gets the environment ssh and
sftp already get, with KEYFUNC_MNEMONIC and KEYFUNC_MNEMONIC_COMMAND
removed. The function that builds that environment moves from the ssh
command group into internal/mnemonic, beside the two variable names,
so all three programs use the one copy. The README says so.

Model: opus-5-5

internal/cli/ssh/install.go

@@ -16,6 +16,7 @@ import (
 
 	"github.com/spf13/cobra"
 	"sneak.berlin/go/keyfunc/internal/cli/signals"
+	"sneak.berlin/go/keyfunc/internal/mnemonic"
 )
 
 // Where the key goes on the host and what the file it arrives in is
@@ -220,7 +221,7 @@ func session(
 
 	//nolint:gosec // the options are the user's own, meant for sftp
 	command := exec.CommandContext(cmd.Context(), "sftp", argv...)
-	command.Env = childEnv()
+	command.Env = mnemonic.ChildEnv()
 	command.Stdin = strings.NewReader(strings.Join(batch, "\n") + "\n")
 	command.Stdout = &said
 	command.Stderr = &said