fix: set XDG_CONFIG_DIRS and XDG_DATA_DIRS defaults in tryGetPamEnvVars (#3121)

## Summary

- Fixes #2970: WaveTerm does not inherit `XDG_CONFIG_DIRS` (and
`XDG_DATA_DIRS`) when snap or other environments strip these variables
and PAM env files do not define them
- In `tryGetPamEnvVars()`, after the existing `XDG_RUNTIME_DIR`
fallback, adds identical fallback logic for `XDG_CONFIG_DIRS` (default:
`/etc/xdg`) and `XDG_DATA_DIRS` (default: `/usr/local/share:/usr/share`)
per the XDG Base Directory Specification
- No behavior change when these vars are already set by PAM env files

## Root Cause

Snap confinement strips several XDG environment variables.
`tryGetPamEnvVars()` already handles `XDG_RUNTIME_DIR` with a sensible
default, but `XDG_CONFIG_DIRS` and `XDG_DATA_DIRS` were left unhandled,
causing child shells to receive empty/unset values.

## Test plan

- [ ] `gofmt -l ./pkg/shellexec/` — no output (clean)
- [ ] `go build ./...` — succeeds
- [ ] On Linux with snap, verify that child shells receive
`XDG_CONFIG_DIRS=/etc/xdg` and
`XDG_DATA_DIRS=/usr/local/share:/usr/share` when the variables are not
set by the desktop environment

Signed-off-by: majiayu000 <1835304752@qq.com>