feat: support fish shell

[kemingy]

• fix feat: support shell("fish") #663

To use fish:

# syntax=v1

def build():
    base(dev=True)
    shell('fish')
    install.conda()
    # install.conda(use_mamba=True)
    install.python()

[Copilot]

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

Comments suppressed due to low confidence (2)

pkg/lang/ir/v1/conda.go:186

• Ensure that the new fish activation scripts for conda are covered by tests.

File(llb.Mkfile(fmt.Sprintf("%s/activate.fish", condaBinDir), 0755, []byte(condaActivateFish)),

pkg/lang/ir/v1/conda.go:210

• Ensure that the new fish activation scripts for micromamba are covered by tests.

File(llb.Mkfile(fmt.Sprintf("%s/activate.fish", condaBinDir), 0755, []byte(mambaActivateFish)),

[kemingy]

I have no idea what happened to the e2e/cli/context_test. Manually running the moby/buildkit:rootless image with envd build succeeded.