build: Adapt to new Go toolchain versioning change in 1.21

[radeksimko]

Currently one test in build is failing because of a minor change in how Go toolchain versions are identified:

E2E_TESTING=1 go test ./build -run=TestGitRevision_vault -v
=== RUN   TestGitRevision_vault
2023/08/23 09:29:40 git_revision.go:91: running vault pre-clone check (timeout: 1m0s)
2023/08/23 09:29:40 git_revision.go:97: vault pre-clone check finished
2023/08/23 09:29:40 git_revision.go:126: cloning vault repository from https://github.com/hashicorp/vault.git to /var/folders/9w/pddq2x656j76vxvsbhcs4nwr0000gq/T/hc-install-build-vault2469906280 (timeout: 5m0s)
2023/08/23 09:33:07 git_revision.go:139: cloning vault finished
2023/08/23 09:33:07 git_revision.go:145: vault repository HEAD is at e4ce8729fde726ed933d231a2eb5e252af229bc1
2023/08/23 09:33:07 git_revision.go:172: building vault (timeout: 15m0s)
2023/08/23 09:33:07 go_build.go:132: attempting to satisfy guessed Go requirement 1.21.0
2023/08/23 09:33:07 install_go_version.go:25: go getting "golang.org/dl/go1.21"
2023/08/23 09:33:09 git_revision.go:174: building of vault finished
    git_revision_test.go:91: unable to get Go 1.21.0: exit status 1
        go: downloading golang.org/dl v0.0.0-20230818220345-55c644201171
        go: module golang.org/dl@upgrade found (v0.0.0-20230818220345-55c644201171), but does not contain package golang.org/dl/go1.21

--- FAIL: TestGitRevision_vault (209.45s)
FAIL
FAIL	github.com/hashicorp/hc-install/build	209.612s
FAIL

See golang/go#62136 (I could not find the PR/CL which actually introduced this change).

I also bumped timeouts again to avoid failures due to slow builds.