*: do not require env.sh

This removes the need to source env.{sh,fish} when working with hscloud.

This is done by:

 1. Implementing a Go library to reliably detect the location of the
    active hscloud checkout. That in turn is enabled by
    BUILD_WORKSPACE_DIRECTORY being now a thing in Bazel.
 2. Creating a tool `hscloud`, with a command `hscloud workspace` that
    returns the workspace path.
 3. Wrapping this tool to be accessible from Python and Bash.
 4. Bumping all users of hscloud_root to use either the Go library or
    one of the two implemented wrappers.

We also drive-by replace tools/install.sh to be a proper sh_binary, and
make it yell at people if it isn't being ran as `bazel run
//tools:install`.

Finally, we also drive-by delete cluster/tools/nixops.sh which was never used.

Change-Id: I7873714319bfc38bbb930b05baa605c5aa36470a
Reviewed-on: https://gerrit.hackerspace.pl/c/hscloud/+/1169
Reviewed-by: informatic <informatic@hackerspace.pl>
diff --git a/env.fish b/env.fish
index 428c5df..02cfa77 100644
--- a/env.fish
+++ b/env.fish
@@ -11,6 +11,8 @@
     set -x PATH $hscloud_path $PATH
 end
 
+# Leftover junk. This should be removed, as env.fish is now optional.
+# Do _not_ add more aliases!
 function gpg-unlock
     echo "test" | gpg2 --sign --batch --no-tty -o /dev/null
 end