docs: update suggestions to use "set" and "unset" environment commands (#525)

Author: zimeg

.github/STYLE_GUIDE.md

@@ -31,7 +31,7 @@ The square brackets surrounding command arguments hint that these are optional:
 
 ```
 USAGE
-  $ slack env add [name] [value] [flags]
+  $ slack env set [name] [value] [flags]
 ```
 
 The angled brackets around arguments hint that these are required:

cmd/env/unset.go

@@ -112,7 +112,7 @@ func runEnvUnsetCommandFunc(clients *shared.ClientFactory, cmd *cobra.Command, a
 				Emoji: "evergreen_tree",
 				Text:  "Environment Unset",
 				Secondary: []string{
-					"The app has no environment variables to remove",
+					"The app has no environment variables to unset",
 				},
 			}))
 			return nil
@@ -141,7 +141,7 @@ func runEnvUnsetCommandFunc(clients *shared.ClientFactory, cmd *cobra.Command, a
 				Emoji: "evergreen_tree",
 				Text:  "Environment Unset",
 				Secondary: []string{
-					"The project has no environment variables to remove",
+					"The project has no environment variables to unset",
 				},
 			}))
 			return nil

cmd/env/unset_test.go

@@ -79,7 +79,7 @@ func Test_Env_RemoveCommand(t *testing.T) {
 				cm.API.AssertNotCalled(t, "RemoveVariable")
 			},
 			ExpectedStdoutOutputs: []string{
-				"The project has no environment variables to remove",
+				"The project has no environment variables to unset",
 			},
 		},
 		"exit without errors when hosted app has zero variables": {
@@ -94,7 +94,7 @@ func Test_Env_RemoveCommand(t *testing.T) {
 				cm.API.AssertNotCalled(t, "RemoveVariable")
 			},
 			ExpectedStdoutOutputs: []string{
-				"The app has no environment variables to remove",
+				"The app has no environment variables to unset",
 			},
 		},
 		"remove a hosted variable using arguments": {

internal/auth/auth.go

@@ -494,7 +494,7 @@ func (c *Client) ResolveAPIHost(ctx context.Context, apiHostFlag string, customA
 		c.io.PrintDebug(
 			ctx,
 			"You're using a custom apihost. Run %s to add it to your app's Run on Slack environment",
-			style.Commandf(fmt.Sprintf("var add SLACK_API_URL %s", apiHost), false),
+			style.Commandf(fmt.Sprintf("env set SLACK_API_URL %s", apiHost), false),
 		)
 	}