fix(agent): filter out GOTRACEBACK=none (#16924)

With the switch to Go 1.24.1, our dogfood workspaces started setting
`GOTRACEBACK=none` in the environment, resulting in missing stacktraces
for users.

This is due to the capability changes we do when
`USE_CAP_NET_ADMIN=true`.

https://github.com/coder/coder/blob/564b387262e5b768c503e5317242d9ab576395d6/provisionersdk/scripts/bootstrap_linux.sh#L60-L76

This most likely triggers a change in securitybits which sets
`_AT_SECURE` for the process.

https://github.com/golang/go/blob/a1ddbdd3ef8b739aab53f20d6ed0a61c3474cf12/src/runtime/os_linux.go#L297-L327

Which in turn triggers secure mode:

https://github.com/golang/go/blob/a1ddbdd3ef8b739aab53f20d6ed0a61c3474cf12/src/runtime/security_unix.go

This should not affect workspaces as template authors can still set the
environment on the agent resource.

See https://pkg.go.dev/runtime#hdr-Security

agent/usershell/usershell_test.go

@@ -43,4 +43,13 @@ func TestGet(t *testing.T) {
 			require.NotEmpty(t, shell)
 		})
 	})
+
+	t.Run("Remove GOTRACEBACK=none", func(t *testing.T) {
+		t.Setenv("GOTRACEBACK", "none")
+		ei := usershell.SystemEnvInfo{}
+		env := ei.Environ()
+		for _, e := range env {
+			require.NotEqual(t, "GOTRACEBACK=none", e)
+		}
+	})
 }