Fix `uxp install` error "couldn't find binding"

[branden]

Description of your changes

This PR makes uxp.installCmd.AfterApply() create an *upbound.Context rather than take it as an arg, since none of its parent commands are binding it.

Fixes #369.

I have:

• [x] Read and followed Upbound's contribution process.
• [x] Run make reviewable to ensure this PR is ready for review.
• [x] Added backport release-x.y labels to auto-backport this PR, as appropriate.

How has this code been tested

I verified that up uxp install makes it past uxp.installCmd.AfterApply():

branden@crateria up % go run ./cmd/up uxp install
up: error: uxp.installCmd.Run(): rendered manifests contain a resource that already exists. Unable to continue with install: ServiceAccount "rbac-manager" in namespace "upbound-system" exists and cannot be imported into the current release: invalid ownership metadata; annotation validation error: key "meta.helm.sh/release-name" must equal "universal-crossplane": current value is "uxp"
exit status 1
branden@crateria up %

The above error is expected since I have a kubeconfig for a spaces cluster with UXP already installed.

[github-actions[bot]]

Successfully created backport PR #371 for release-0.19.