docker buildx inspect --bootstrap can return inactive nodes

[jedevc]

Found while working on #1934.

Version: github.com/docker/buildx 0.11.0 687feca9e8dcd1534ac4c026bc4db5a49de0dd6e

Reproducer:

$ docker buildx create --driver=remote --name dev tcp://localhost:1234
$ docker buildx inspect dev --bootstrap # then start a buildkit server on localhost:1234 in another terminal
[+] Building 0.0s (0/0)                                                                                                                                              
Name:          dev
Driver:        remote
Last Activity: 2023-07-05 09:56:56 +0000 UTC

Nodes:
Name:      dev0
Endpoint:  tcp://localhost:1234
Status:    inactive
Platforms: 

The expected output is instead:

$ docker buildx inspect dev --bootstrap
Name:          dev
Driver:        remote
Last Activity: 2023-07-05 09:56:56 +0000 UTC

Nodes:
Name:      dev0
Endpoint:  tcp://localhost:1234
Status:    running
Buildkit:  v0.12.0-rc1-4-g66a4b9421.m
Platforms: linux/amd64, linux/amd64/v2, linux/amd64/v3, linux/arm64, linux/riscv64, linux/ppc64le, linux/s390x, linux/386, linux/mips64le, linux/mips64, linux/arm/v7, linux/arm/v6
Labels:
 org.mobyproject.buildkit.worker.executor:         oci
 org.mobyproject.buildkit.worker.hostname:         dc141ad6e142

The node is correctly bootstrapped (by waiting for it to be ready), but the output is incorrect and shows the builder as inactive.

This does appear to be slightly racy.