chore: respect PKG_VSN in pkg-vsn.sh

[id]

No changes in logic. Right now the same happens on line #110 in the current version: if PKG_VSN is set, and --long is not given, we just return PKG_VSN.

In the current version though, even if you set PKG_VSN, the script will still attempt to extract version from git. This fails to produce correct version if one tries to build emqx from source code archive (with no .git), and consequently fails the build.

[coveralls]

Pull Request Test Coverage Report for Build 3176349197

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 34 unchanged lines in 15 files lost coverage.
• Overall coverage decreased (-0.02%) to 77.968%

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Files with Coverage Reduction	New Missed Lines	%
apps/emqx_authz/src/emqx_authz_schema.erl	1	90.28%
apps/emqx_conf/src/emqx_conf_schema.erl	1	92.31%
apps/emqx_gateway/src/mqttsn/emqx_sn_channel.erl	1	72.92%
apps/emqx_management/src/emqx_mgmt_api_trace.erl	1	80.0%
apps/emqx/src/emqx_broker.erl	1	81.87%
apps/emqx/src/emqx_cm.erl	1	88.99%
apps/emqx/src/emqx_limiter/src/emqx_limiter_server.erl	1	89.08%
apps/emqx/src/emqx_listeners.erl	1	85.12%
apps/emqx/src/emqx_ws_connection.erl	1	79.28%
apps/emqx_gateway/src/exproto/emqx_exproto_gcli.erl	3	48.78%

Totals
Change from base Build 3152209479:	-0.02%
Covered Lines:	20723
Relevant Lines:	26579

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💛 - Coveralls