Consistently generate build tags on newer Make

With Make 4.3+, the empty whitespace does not seem to work as
originally intended. This causes build tags to be "netgo ledger,"
on Ubuntu 22.04 and other systems that include the newer Make
version. The build tags were intended as "netgo,ledger" which
can be observed on Make 4.2 (shipped with Ubuntu 20.04).

This change swaps out the `+=` use in favor of an explicit `:=`.
Ref: https://www.gnu.org/software/make/manual/html_node/Appending.html

Closes cosmos#2017.

Makefile

@@ -55,7 +55,7 @@ build_tags += $(BUILD_TAGS)
 build_tags := $(strip $(build_tags))
 
 whitespace :=
-whitespace += $(whitespace)
+whitespace := $(whitespace) $(whitespace)
 comma := ,
 build_tags_comma_sep := $(subst $(whitespace),$(comma),$(build_tags))