Boxplot boxmode "group" doesn't work for subplots

**Describe the bug**

Assume we have a series of boxplots:
```jl
julia> boxplots = [box(x=repeat(1:5,10), y=vec(randn(5, 10))) for _ in 1:3]
```
If I put these traces inside a plot, with `boxmode` set to `"group"`, it works as expected:

```jl
julia> plot(boxplots, Layout(boxmode="group"))
```


But if I put the plot inside a subplot grid, the `boxmode` parameter is “forgotten”:

```jl
julia> vcat(plot(boxplots, Layout(boxmode="group")))
```


It is similarly broken, obviously, when I put multiple subplots in the grid.

**Version info**

```jl
julia> versioninfo()
Julia Version 1.7.3
Commit 742b9abb4d (2022-05-06 12:58 UTC)
Platform Info:
OS: Linux (x86_64-pc-linux-gnu)
CPU: Intel(R) Core(TM) i7-10510U CPU @ 1.80GHz
WORD_SIZE: 64
LIBM: libopenlibm
LLVM: libLLVM-12.0.1 (ORCJIT, skylake)
Environment:
JULIA_PKG_USE_CLI_GIT = true

julia> using Pkg; pkg"status"
Status `~/.julia/environments/v1.7/Project.toml`
[6e4b80f9] BenchmarkTools v1.3.1
[ad839575] Blink v0.12.5
[a2441757] Coverage v1.6.0
[31a5f54b] Debugger v0.7.6
[54eb57ff] InteractiveCodeSearch v0.4.2
[01369628] MyUtils v0.4.1 `~/CI/MyUtils.jl`
[5fb14364] OhMyREPL v0.5.12
[9b87118b] PackageCompiler v2.0.9
[f0f68f2c] PlotlyJS v0.18.8
[c46f51b8] ProfileView v1.5.1
[e0db7c4e] ReTest v0.3.2
[295af30f] Revise v3.4.0
```