Use correct PowerShell syntax for environment variables
In PowerShell, environment variables must be referenced as
${env:XXX} (or $env:XXX), not ${XXX}.
Since VCPKG_VERSION is defined as an environment variable,
${VCPKG_VERSION} expands to an empty string, producing an
invalid download URL. Likewise, ${TRIPLET} expands to an
empty string and results in an incorrect ZIP filename.
This issue appears to have been introduced by
"Pedantic zizmor checks"
(2654c9dd5637fee6cacf8ca077dbbcb07e796ecc).