buildtools-tarball: add envvars into BB_ENV_PASSTHROUGH_ADDITIONS

buildtools-tarball: add envvars into BB_ENV_PASSTHROUGH_ADDITIONS

Here is one testcase:
For recipe tensorflow-lite-host-tools_2.18.0.bb, refer [1],
do_configure[network] = "1"
and it will git clone some repos in CMakeLists.txt

When buildtools is used and nativesdk-git is installed into sdk,
do_configure failed with error:
[1/9] Performing download step (git clone) for 'protobuf-populate'
Cloning into 'protobuf'...
fatal: unable to access 'https://github.com/protocolbuffers/protobuf/': error setting certificate file: /usr/local/oe-sdk-hardcoded-buildpath/sysroots/x86_64-wrlinuxsdk-linux/etc/ssl/certs/ca-certificates.crt

Fix by adding GIT_SSL_CAINFO in BB_ENV_PASSTHROUGH_ADDITIONS, so that
user can export GIT_SSL_CAINFO=${GIT_SSL_CAINFO} in their
do_configure:prepend() to fix above do_configure failure

CURL_CA_BUNDLE and REQUESTS_CA_BUNDLE is similar envvars, so all add
into BB_ENV_PASSTHROUGH_ADDITIONS

[1] https://github.com/nxp-imx/meta-imx/blob/styhead-6.12.3-1.0.0/meta-imx-ml/recipes-libraries/tensorflow-lite/tensorflow-lite-host-tools_2.18.0.bb

Signed-off-by: Changqing Li <changqing.li@windriver.com>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>