The text on button can't be show correctly when use imported font

[aimin-git]

Describe the bug:

The set on button can't be show correctly when use imported font
The text in label can be shown correctly but the text on button can't.

To Reproduce:

Steps to reproduce the behaviour:

1. Go to '...'
2. Click on '....'
3. Scroll down to '....'
4. See error

Screenshots:

Example code:

func init() {
fontPaths := findfont.List()
for _, path := range fontPaths {
fmt.Println(path)
//楷体:simkai.ttf
//黑体:simhei.ttf
//PingFang-Regular.ttf
if strings.Contains(path, "PingFang-Regular.ttf") {
os.Setenv("FYNE_FONT", path)
break
}
}
fmt.Println("=============")
}

Device (please complete the following information):

• OS: Windows10
• Version: <!-- [e.g. 5.10.2, 10.13 High Sierra or 14.2]
• Go version: 1.16.6
• Fyne version: Fyne 2.1.0

[andydotxyz]

You have not provided any information about Fyne version, etc - please fill in the bug report template so we can help.

[andydotxyz]

I expect you see these errors in the console as well:

FYNE_FONT=~/Downloads/PingFang\ Regular.ttf go run .
2021/12/06 19:34:00 Fyne error:  Error loading specified font
2021/12/06 19:34:00   Cause: open /Users/andy/Downloads/PingFang Bold.ttf: no such file or directory
2021/12/06 19:34:00   At: /Users/andy/Code/fyne/theme/theme.go:626
2021/12/06 19:34:00 Fyne error:  Error loading specified font
2021/12/06 19:34:00   Cause: open /Users/andy/Downloads/PingFang Italic.ttf: no such file or directory
2021/12/06 19:34:00   At: /Users/andy/Code/fyne/theme/theme.go:626
2021/12/06 19:34:00 Fyne error:  Error loading specified font
2021/12/06 19:34:00   Cause: open /Users/andy/Downloads/PingFang BoldItalic.ttf: no such file or directory
2021/12/06 19:34:00   At: /Users/andy/Code/fyne/theme/theme.go:626

Clearly it expects other fonts to be available at the moment.
We could probably fall back to regular if the others are not found.