ci: avoid moving the build directory on GHA

While waiting for a PR job to start testing my code, I noticed the
symlink-build-dir step took 10 minutes to complete, so I investigated
what caused that.

It seems like something changed in the build environment between version
20200901.1 (where the step took 45 seconds) and version 20200908.1
(where the step took 10 minutes). At the time of writing this commit,
the rust-lang organization is on vertsion 20200908.1, while the
rust-lang-ci organization is at version 20200901.1 (and is not affected
by this yet).

There is no need for this step anymore on GHA, as our XL builders got an
increase in the root paritition size, so this commit removes the code
that moved stuff around on GHA (while keeping it on Azure).

For the record, at the time of writing this, the disk situation is:

Filesystem      Size  Used Avail Use% Mounted on
/dev/sda1       667G   60G  607G   9% /
/dev/sdb1       110G  4.1G  101G   4% /mnt

0 files changed, 0 insertions, 0 deletions