ci: don't move the build to C: on GitHub Actions

On Azure Pipeliones, the C: filesystem is huge with a lot of free space,
while D: is small. By default builds happened in D:, so we added a
script to symlink the big directories to C:, granting us more space.

    Filesystem  Size  Used Avail Use%
    C:          256G  143G  114G  56%
    D:           14G  2.0G   13G  15%

On GitHub Actions instead C: is almost full, and we have a lot of free
space on D:, where the build happens.

    Filesystem  Size  Used Avail Use%
    C:          128G  114G   15G  89%
    D:          56G   4.8G   52G   9%

This commit stops creating the symlink on GitHub Actions, fixing the out
of disk space errors we were seeing on some Windows builders.

Author: pietroalbini

src/ci/scripts/symlink-build-dir.sh

@@ -9,7 +9,7 @@ IFS=$'\n\t'
 
 source "$(cd "$(dirname "$0")" && pwd)/../shared.sh"
 
-if isWindows; then
+if isWindows && isAzurePipelines; then
     cmd //c "mkdir c:\\MORE_SPACE"
     cmd //c "mklink /J build c:\\MORE_SPACE"
 fi